Appendix B Errors, Error Codes, and Common Problems
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Appendix B Errors, Error Codes, and Common Problems

Table of Contents

B.1 Sources of Error Information
B.2 Types of Error Values
B.3 Server Error Codes and Messages
B.4 Client Error Codes and Messages
B.5 Problems and Common Errors
B.5.1 How to Determine What Is Causing a Problem
B.5.2 Common Errors When Using MySQL Programs
B.5.3 Administration-Related Issues
B.5.4 Query-Related Issues
B.5.5 Optimizer-Related Issues
B.5.6 Table Definition-Related Issues
B.5.7 Known Issues in MySQL

This appendix lists common problems and errors that may occur and potential resolutions, in addition to listing the errors that may appear when you call MySQL from any host language. The first section covers problems and resolutions. Detailed information on errors is provided: One list displays server error messages. Another list displays client program messages.

B.1 Sources of Error Information

There are several sources of error information in MySQL:

Descriptions of server and client errors are provided later in this Appendix. For information about errors related to InnoDB, see Section 15.19.4, “InnoDB Error Handling”.

B.2 Types of Error Values

When an error occurs in MySQL, the server returns two types of error values:

  • A MySQL-specific error code. This value is numeric. It is not portable to other database systems.

  • An SQLSTATE value. The value is a five-character string (for example, '42S02'). The values are taken from ANSI SQL and ODBC and are more standardized.

A message string that provides a textual description of the error is also available.

When an error occurs, the MySQL error code, SQLSTATE value, and message string are available using C API functions:

For prepared statements, the corresponding error functions are mysql_stmt_errno(), mysql_stmt_sqlstate(), and mysql_stmt_error(). All error functions are described in Section 25.8, “MySQL C API”.

The number of errors, warnings, and notes for the previous statement can be obtained by calling mysql_warning_count(). See Section 25.8.7.78, “mysql_warning_count()”.

The first two characters of an SQLSTATE value indicate the error class:

  • Class = '00' indicates success.

  • Class = '01' indicates a warning.

  • Class = '02' indicates “not found.” This is relevant within the context of cursors and is used to control what happens when a cursor reaches the end of a data set. This condition also occurs for SELECT ... INTO var_list statements that retrieve no rows.

  • Class > '02' indicates an exception.

B.3 Server Error Codes and Messages

MySQL programs have access to several types of error information when the server returns an error. For example, the mysql client program displays errors using the following format: 

shell> SELECT * FROM no_such_table;
ERROR 1146 (42S02): Table 'test.no_such_table' doesn't exist

The message displayed contains three types of information:

  • A numeric error code (1146). This number is MySQL-specific and is not portable to other database systems.

  • A five-character SQLSTATE value ('42S02'). The values are taken from ANSI SQL and ODBC and are more standardized. Not all MySQL error numbers have corresponding SQLSTATE values. In these cases, 'HY000' (general error) is used.

  • A message string that provides a textual description of the error.

For error checking, use error codes, not error messages. Error messages do not change often, but it is possible. Also if the database administrator changes the language setting, that affects the language of error messages.

Error codes are stable across GA releases of a given MySQL series. Before a series reaches GA status, new codes may still be under development and subject to change.

Server error information comes from the following source files. For details about the way that error information is defined, see the MySQL Internals Manual.

  • Error message information is listed in the share/errmsg.txt file. %d and %s represent numbers and strings, respectively, that are substituted into the Message values when they are displayed.

  • The Error values listed in share/errmsg.txt are used to generate the definitions in the include/mysqld_error.h and include/mysqld_ername.h MySQL source files.

  • The SQLSTATE values listed in share/errmsg.txt are used to generate the definitions in the include/sql_state.h MySQL source file.

Because updates are frequent, it is possible that those files will contain additional error information not listed here.

B.4 Client Error Codes and Messages

Client error information comes from the following source files:

  • The Error values and the symbols in parentheses correspond to definitions in the include/errmsg.h MySQL source file.

  • The Message values correspond to the error messages that are listed in the libmysql/errmsg.c file. %d and %s represent numbers and strings, respectively, that are substituted into the messages when they are displayed.

Because updates are frequent, it is possible that those files will contain additional error information not listed here.

B.5 Problems and Common Errors

B.5.1 How to Determine What Is Causing a Problem
B.5.2 Common Errors When Using MySQL Programs
B.5.3 Administration-Related Issues
B.5.4 Query-Related Issues
B.5.5 Optimizer-Related Issues
B.5.6 Table Definition-Related Issues
B.5.7 Known Issues in MySQL

This section lists some common problems and error messages that you may encounter. It describes how to determine the causes of the problems and what to do to solve them.

B.5.1 How to Determine What Is Causing a Problem

When you run into a problem, the first thing you should do is to find out which program or piece of equipment is causing it:

  • If you have one of the following symptoms, then it is probably a hardware problems (such as memory, motherboard, CPU, or hard disk) or kernel problem:

    • The keyboard does not work. This can normally be checked by pressing the Caps Lock key. If the Caps Lock light does not change, you have to replace your keyboard. (Before doing this, you should try to restart your computer and check all cables to the keyboard.)

    • The mouse pointer does not move.

    • The machine does not answer to a remote machine's pings.

    • Other programs that are not related to MySQL do not behave correctly.

    • Your system restarted unexpectedly. (A faulty user-level program should never be able to take down your system.)

    In this case, you should start by checking all your cables and run some diagnostic tool to check your hardware! You should also check whether there are any patches, updates, or service packs for your operating system that could likely solve your problem. Check also that all your libraries (such as glibc) are up to date.

    It is always good to use a machine with ECC memory to discover memory problems early.

  • If your keyboard is locked up, you may be able to recover by logging in to your machine from another machine and executing kbd_mode -a.

  • Please examine your system log file (/var/log/messages or similar) for reasons for your problem. If you think the problem is in MySQL, you should also examine MySQL's log files. See Section 6.4, “MySQL Server Logs”.

  • If you do not think you have hardware problems, you should try to find out which program is causing problems. Try using top, ps, Task Manager, or some similar program, to check which program is taking all CPU or is locking the machine.

  • Use top, df, or a similar program to check whether you are out of memory, disk space, file descriptors, or some other critical resource.

  • If the problem is some runaway process, you can always try to kill it. If it does not want to die, there is probably a bug in the operating system.

If after you have examined all other possibilities and you have concluded that the MySQL server or a MySQL client is causing the problem, it is time to create a bug report for our mailing list or our support team. In the bug report, try to give a very detailed description of how the system is behaving and what you think is happening. You should also state why you think that MySQL is causing the problem. Take into consideration all the situations in this chapter. State any problems exactly how they appear when you examine your system. Use the “copy and paste” method for any output and error messages from programs and log files.

Try to describe in detail which program is not working and all symptoms you see. We have in the past received many bug reports that state only “the system does not work.” This provides us with no information about what could be the problem.

If a program fails, it is always useful to know the following information:

  • Has the program in question made a segmentation fault (did it dump core)?

  • Is the program taking up all available CPU time? Check with top. Let the program run for a while, it may simply be evaluating something computationally intensive.

  • If the mysqld server is causing problems, can you get any response from it with mysqladmin -u root ping or mysqladmin -u root processlist?

  • What does a client program say when you try to connect to the MySQL server? (Try with mysql, for example.) Does the client jam? Do you get any output from the program?

When sending a bug report, you should follow the outline described in Section 1.7, “How to Report Bugs or Problems”.

B.5.2 Common Errors When Using MySQL Programs

B.5.2.1 Access denied
B.5.2.2 Can't connect to [local] MySQL server
B.5.2.3 Lost connection to MySQL server
B.5.2.4 Client does not support authentication protocol
B.5.2.5 Password Fails When Entered Interactively
B.5.2.6 Host 'host_name' is blocked
B.5.2.7 Too many connections
B.5.2.8 Out of memory
B.5.2.9 MySQL server has gone away
B.5.2.10 Packet Too Large
B.5.2.11 Communication Errors and Aborted Connections
B.5.2.12 The table is full
B.5.2.13 Can't create/write to file
B.5.2.14 Commands out of sync
B.5.2.15 Ignoring user
B.5.2.16 Table 'tbl_name' doesn't exist
B.5.2.17 Can't initialize character set
B.5.2.18 File Not Found and Similar Errors
B.5.2.19 Table-Corruption Issues

This section lists some errors that users frequently encounter when running MySQL programs. Although the problems show up when you try to run client programs, the solutions to many of the problems involves changing the configuration of the MySQL server.

B.5.2.1 Access denied

An Access denied error can have many causes. Often the problem is related to the MySQL accounts that the server permits client programs to use when connecting. See Section 7.2, “The MySQL Access Privilege System”, and Section 7.2.7, “Troubleshooting Problems Connecting to MySQL”.

B.5.2.2 Can't connect to [local] MySQL server

A MySQL client on Unix can connect to the mysqld server in two different ways: By using a Unix socket file to connect through a file in the file system (default /tmp/mysql.sock), or by using TCP/IP, which connects through a port number. A Unix socket file connection is faster than TCP/IP, but can be used only when connecting to a server on the same computer. A Unix socket file is used if you do not specify a host name or if you specify the special host name localhost.

If the MySQL server is running on Windows, you can connect using TCP/IP. If the server is started with the --enable-named-pipe option, you can also connect with named pipes if you run the client on the host where the server is running. The name of the named pipe is MySQL by default. If you do not give a host name when connecting to mysqld, a MySQL client first tries to connect to the named pipe. If that does not work, it connects to the TCP/IP port. You can force the use of named pipes on Windows by using . as the host name.

The error (2002) Can't connect to ... normally means that there is no MySQL server running on the system or that you are using an incorrect Unix socket file name or TCP/IP port number when trying to connect to the server. You should also check that the TCP/IP port you are using has not been blocked by a firewall or port blocking service.

The error (2003) Can't connect to MySQL server on 'server' (10061) indicates that the network connection has been refused. You should check that there is a MySQL server running, that it has network connections enabled, and that the network port you specified is the one configured on the server.

Start by checking whether there is a process named mysqld running on your server host. (Use ps xa | grep mysqld on Unix or the Task Manager on Windows.) If there is no such process, you should start the server. See Section 2.10.2, “Starting the Server”.

If a mysqld process is running, you can check it by trying the following commands. The port number or Unix socket file name might be different in your setup. host_ip represents the IP address of the machine where the server is running. 

shell> mysqladmin version
shell> mysqladmin variables
shell> mysqladmin -h `hostname` version variables
shell> mysqladmin -h `hostname` --port=3306 version
shell> mysqladmin -h host_ip version
shell> mysqladmin --protocol=SOCKET --socket=/tmp/mysql.sock version

Note the use of backticks rather than forward quotation marks with the hostname command; these cause the output of hostname (that is, the current host name) to be substituted into the mysqladmin command. If you have no hostname command or are running on Windows, you can manually type the host name of your machine (without backticks) following the -h option. You can also try -h 127.0.0.1 to connect with TCP/IP to the local host.

Make sure that the server has not been configured to ignore network connections or (if you are attempting to connect remotely) that it has not been configured to listen only locally on its network interfaces. If the server was started with --skip-networking, it will not accept TCP/IP connections at all. If the server was started with --bind-address=127.0.0.1, it will listen for TCP/IP connections only locally on the loopback interface and will not accept remote connections.

Check to make sure that there is no firewall blocking access to MySQL. Your firewall may be configured on the basis of the application being executed, or the port number used by MySQL for communication (3306 by default). Under Linux or Unix, check your IP tables (or similar) configuration to ensure that the port has not been blocked. Under Windows, applications such as ZoneAlarm or the Windows XP personal firewall may need to be configured not to block the MySQL port.

Here are some reasons the Can't connect to local MySQL server error might occur:

  • mysqld is not running on the local host. Check your operating system's process list to ensure the mysqld process is present.

  • You're running a MySQL server on Windows with many TCP/IP connections to it. If you're experiencing that quite often your clients get that error, you can find a workaround here: Section B.5.2.2.1, “Connection to MySQL Server Failing on Windows”.

  • Someone has removed the Unix socket file that mysqld uses (/tmp/mysql.sock by default). For example, you might have a cron job that removes old files from the /tmp directory. You can always run mysqladmin version to check whether the Unix socket file that mysqladmin is trying to use really exists. The fix in this case is to change the cron job to not remove mysql.sock or to place the socket file somewhere else. See Section B.5.3.6, “How to Protect or Change the MySQL Unix Socket File”.

  • You have started the mysqld server with the --socket=/path/to/socket option, but forgotten to tell client programs the new name of the socket file. If you change the socket path name for the server, you must also notify the MySQL clients. You can do this by providing the same --socket option when you run client programs. You also need to ensure that clients have permission to access the mysql.sock file. To find out where the socket file is, you can do: 

    shell> netstat -ln | grep mysql

    See Section B.5.3.6, “How to Protect or Change the MySQL Unix Socket File”.

  • You are using Linux and one server thread has died (dumped core). In this case, you must kill the other mysqld threads (for example, with kill) before you can restart the MySQL server. See Section B.5.3.3, “What to Do If MySQL Keeps Crashing”.

  • The server or client program might not have the proper access privileges for the directory that holds the Unix socket file or the socket file itself. In this case, you must either change the access privileges for the directory or socket file so that the server and clients can access them, or restart mysqld with a --socket option that specifies a socket file name in a directory where the server can create it and where client programs can access it.

If you get the error message Can't connect to MySQL server on some_host, you can try the following things to find out what the problem is:

  • Check whether the server is running on that host by executing telnet some_host 3306 and pressing the Enter key a couple of times. (3306 is the default MySQL port number. Change the value if your server is listening to a different port.) If there is a MySQL server running and listening to the port, you should get a response that includes the server's version number. If you get an error such as telnet: Unable to connect to remote host: Connection refused, then there is no server running on the given port.

  • If the server is running on the local host, try using mysqladmin -h localhost variables to connect using the Unix socket file. Verify the TCP/IP port number that the server is configured to listen to (it is the value of the port variable.)

  • If you are running under Linux and Security-Enhanced Linux (SELinux) is enabled, make sure you have disabled SELinux protection for the mysqld process.

B.5.2.2.1 Connection to MySQL Server Failing on Windows

When you're running a MySQL server on Windows with many TCP/IP connections to it, and you're experiencing that quite often your clients get a Can't connect to MySQL server error, the reason might be that Windows does not allow for enough ephemeral (short-lived) ports to serve those connections.

The purpose of TIME_WAIT is to keep a connection accepting packets even after the connection has been closed. This is because Internet routing can cause a packet to take a slow route to its destination and it may arrive after both sides have agreed to close. If the port is in use for a new connection, that packet from the old connection could break the protocol or compromise personal information from the original connection. The TIME_WAIT delay prevents this by ensuring that the port cannot be reused until after some time has been permitted for those delayed packets to arrive.

It is safe to reduce TIME_WAIT greatly on LAN connections because there is little chance of packets arriving at very long delays, as they could through the Internet with its comparatively large distances and latencies.

Windows permits ephemeral (short-lived) TCP ports to the user. After any port is closed it will remain in a TIME_WAIT status for 120 seconds. The port will not be available again until this time expires. The default range of port numbers depends on the version of Windows, with a more limited number of ports in older versions:

  • Windows through Server 2003: Ports in range 1025–5000

  • Windows Vista, Server 2008, and newer: Ports in range 49152–65535

With a small stack of available TCP ports (5000) and a high number of TCP ports being open and closed over a short period of time along with the TIME_WAIT status you have a good chance for running out of ports. There are two ways to address this problem:

  • Reduce the number of TCP ports consumed quickly by investigating connection pooling or persistent connections where possible

  • Tune some settings in the Windows registry (see below)

Important

The following procedure involves modifying the Windows registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore it if a problem occurs. For information about how to back up, restore, and edit the registry, view the following article in the Microsoft Knowledge Base: http://support.microsoft.com/kb/256986/EN-US/.

  1. Start Registry Editor (Regedt32.exe).

  2. Locate the following key in the registry: 

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

  3. On the Edit menu, click Add Value, and then add the following registry value: 

    Value Name: MaxUserPort
Data Type: REG_DWORD
Value: 65534

    This sets the number of ephemeral ports available to any user. The valid range is between 5000 and 65534 (decimal). The default value is 0x1388 (5000 decimal).

  4. On the Edit menu, click Add Value, and then add the following registry value: 

    Value Name: TcpTimedWaitDelay
Data Type: REG_DWORD
Value: 30

    This sets the number of seconds to hold a TCP port connection in TIME_WAIT state before closing. The valid range is between 30 and 300 decimal, although you may wish to check with Microsoft for the latest permitted values. The default value is 0x78 (120 decimal).

  5. Quit Registry Editor.

  6. Reboot the machine.

Note: Undoing the above should be as simple as deleting the registry entries you've created.

B.5.2.3 Lost connection to MySQL server

There are three likely causes for this error message.

Usually it indicates network connectivity trouble and you should check the condition of your network if this error occurs frequently. If the error message includes “during query,” this is probably the case you are experiencing.

Sometimes the “during query” form happens when millions of rows are being sent as part of one or more queries. If you know that this is happening, you should try increasing net_read_timeout from its default of 30 seconds to 60 seconds or longer, sufficient for the data transfer to complete.

More rarely, it can happen when the client is attempting the initial connection to the server. In this case, if your connect_timeout value is set to only a few seconds, you may be able to resolve the problem by increasing it to ten seconds, perhaps more if you have a very long distance or slow connection. You can determine whether you are experiencing this more uncommon cause by using SHOW GLOBAL STATUS LIKE 'Aborted_connects'. It will increase by one for each initial connection attempt that the server aborts. You may see “reading authorization packet” as part of the error message; if so, that also suggests that this is the solution that you need.

If the cause is none of those just described, you may be experiencing a problem with BLOB values that are larger than max_allowed_packet, which can cause this error with some clients. Sometime you may see an ER_NET_PACKET_TOO_LARGE error, and that confirms that you need to increase max_allowed_packet.

B.5.2.4 Client does not support authentication protocol

Note

The information in this section applies only before MySQL 5.7.5. Support for pre-4.1 password hashes is removed in MySQL 5.7.5. This includes removal of the mysql_old_password authentication plugin and the OLD_PASSWORD() function. Also, secure_auth cannot be disabled, and old_passwords cannot be set to 1.

The current implementation of the authentication protocol uses a password hashing algorithm that is incompatible with that used by older (pre-4.1) clients. Attempts to connect to a 4.1 or higher server with an older client may fail with the following message: 

shell> mysql
Client does not support authentication protocol requested
by server; consider upgrading MySQL client

To deal with this problem, the preferred solution is to upgrade all client programs to use a 4.1.1 or higher client library. If that is not possible, use one of the following approaches:

  • To connect to the server with a pre-4.1 client program, use an account that still has a pre-4.1-style password.

  • Reset the password to pre-4.1 style for each user that needs to use a pre-4.1 client program. This can be done using the SET PASSWORD statement and the OLD_PASSWORD() function. It is also necessary to first ensure that the authentication plugin for the account is mysql_old_password: 

    mysql> UPDATE mysql.user SET plugin = 'mysql_old_password'
mysql> WHERE User = 'some_user' AND Host = 'some_host';
mysql> FLUSH PRIVILEGES;
mysql> SET PASSWORD FOR
    -> 'some_user'@'some_host' = OLD_PASSWORD('new_password');

    Substitute the password you want to use for “new_password” in the preceding example. MySQL cannot tell you what the original password was, so you need to pick a new one.

  • Tell the server to use the older password hashing algorithm by default:

    1. Start mysqld with the old_passwords system variable set to 1.

    2. Assign an old-format password to each account that has had its password updated to the longer 4.1 format. You can identify these accounts with the following query: 

      mysql> SELECT Host, User, Password FROM mysql.user
    -> WHERE LENGTH(Password) > 16;

      For each account record displayed by the query, use the Host and User values and assign a password using one of the methods described previously.

The Client does not support authentication protocol error also can occur if multiple versions of MySQL are installed but client programs are dynamically linked and link to an older library. Make sure that clients use the most recent library version with which they are compatible. The procedure to do this will depend on your system.

Note

The PHP mysql extension does not support the authentication protocol in MySQL 4.1.1 and higher. This is true regardless of the PHP version being used. If you wish to use the mysql extension with MySQL 4.1 or higher, you may need to follow one of the options discussed above for configuring MySQL to work with old clients. The mysqli extension (stands for "MySQL, Improved"; added in PHP 5) is compatible with the improved password hashing employed in MySQL 4.1 and higher, and no special configuration of MySQL need be done to use this MySQL client library. For more information about the mysqli extension, see http://php.net/mysqli.

For additional background on password hashing and authentication, see Section 7.1.2.4, “Password Hashing in MySQL”.

B.5.2.5 Password Fails When Entered Interactively

MySQL client programs prompt for a password when invoked with a --password or -p option that has no following password value: 

shell> mysql -u user_name -p
Enter password:

On some systems, you may find that your password works when specified in an option file or on the command line, but not when you enter it interactively at the Enter password: prompt. This occurs when the library provided by the system to read passwords limits password values to a small number of characters (typically eight). That is a problem with the system library, not with MySQL. To work around it, change your MySQL password to a value that is eight or fewer characters long, or put your password in an option file.

B.5.2.6 Host 'host_name' is blocked

If the following error occurs, it means that mysqld has received many connection requests from the given host that were interrupted in the middle: 

Host 'host_name' is blocked because of many connection errors.
Unblock with 'mysqladmin flush-hosts'

The value of the max_connect_errors system variable determines how many successive interrupted connection requests are permitted. (See Section 6.1.4, “Server System Variables”.) After max_connect_errors failed requests without a successful connection, mysqld assumes that something is wrong (for example, that someone is trying to break in), and blocks the host from further connections until you issue a FLUSH HOSTS statement or execute a mysqladmin flush-hosts command.

By default, mysqld blocks a host after 100 connection errors. You can adjust the value by setting max_connect_errors at server startup: 

shell> mysqld_safe --max_connect_errors=10000 &

The value can also be set at runtime: 

mysql> SET GLOBAL max_connect_errors=10000;

If you get the Host 'host_name' is blocked error message for a given host, you should first verify that there is nothing wrong with TCP/IP connections from that host. If you are having network problems, it does you no good to increase the value of the max_connect_errors variable.

B.5.2.7 Too many connections

If you get a Too many connections error when you try to connect to the mysqld server, this means that all available connections are in use by other clients.

The number of connections permitted is controlled by the max_connections system variable. The default value is 151 to improve performance when MySQL is used with the Apache Web server. (Previously, the default was 100.) If you need to support more connections, you should set a larger value for this variable.

mysqld actually permits max_connections+1 clients to connect. The extra connection is reserved for use by accounts that have the SUPER privilege. By granting the SUPER privilege to administrators and not to normal users (who should not need it), an administrator can connect to the server and use SHOW PROCESSLIST to diagnose problems even if the maximum number of unprivileged clients are connected. See Section 14.7.5.29, “SHOW PROCESSLIST Syntax”.

The maximum number of connections MySQL can support depends on the quality of the thread library on a given platform, the amount of RAM available, how much RAM is used for each connection, the workload from each connection, and the desired response time. Linux or Solaris should be able to support at 500 to 1000 simultaneous connections routinely and as many as 10,000 connections if you have many gigabytes of RAM available and the workload from each is low or the response time target undemanding. Windows is limited to (open tables × 2 + open connections) < 2048 due to the Posix compatibility layer used on that platform.

Increasing open-files-limit may be necessary. Also see Section 2.5, “Installing MySQL on Linux”, for how to raise the operating system limit on how many handles can be used by MySQL.

B.5.2.8 Out of memory

If you issue a query using the mysql client program and receive an error like the following one, it means that mysql does not have enough memory to store the entire query result: 

mysql: Out of memory at line 42, 'malloc.c'
mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k)
ERROR 2008: MySQL client ran out of memory

To remedy the problem, first check whether your query is correct. Is it reasonable that it should return so many rows? If not, correct the query and try again. Otherwise, you can invoke mysql with the --quick option. This causes it to use the mysql_use_result() C API function to retrieve the result set, which places less of a load on the client (but more on the server).

B.5.2.9 MySQL server has gone away

This section also covers the related Lost connection to server during query error.

The most common reason for the MySQL server has gone away error is that the server timed out and closed the connection. In this case, you normally get one of the following error codes (which one you get is operating system-dependent).

Error CodeDescription
CR_SERVER_GONE_ERRORThe client couldn't send a question to the server.
CR_SERVER_LOSTThe client didn't get an error when writing to the server, but it didn't get a full answer (or any answer) to the question.

By default, the server closes the connection after eight hours if nothing has happened. You can change the time limit by setting the wait_timeout variable when you start mysqld. See Section 6.1.4, “Server System Variables”.

If you have a script, you just have to issue the query again for the client to do an automatic reconnection. This assumes that you have automatic reconnection in the client enabled (which is the default for the mysql command-line client).

Some other common reasons for the MySQL server has gone away error are:

  • You (or the db administrator) has killed the running thread with a KILL statement or a mysqladmin kill command.

  • You tried to run a query after closing the connection to the server. This indicates a logic error in the application that should be corrected.

  • A client application running on a different host does not have the necessary privileges to connect to the MySQL server from that host.

  • You got a timeout from the TCP/IP connection on the client side. This may happen if you have been using the commands: mysql_options(..., MYSQL_OPT_READ_TIMEOUT,...) or mysql_options(..., MYSQL_OPT_WRITE_TIMEOUT,...). In this case increasing the timeout may help solve the problem.

  • You have encountered a timeout on the server side and the automatic reconnection in the client is disabled (the reconnect flag in the MYSQL structure is equal to 0).

  • You are using a Windows client and the server had dropped the connection (probably because wait_timeout expired) before the command was issued.

    The problem on Windows is that in some cases MySQL does not get an error from the OS when writing to the TCP/IP connection to the server, but instead gets the error when trying to read the answer from the connection.

    The solution to this is to either do a mysql_ping() on the connection if there has been a long time since the last query (this is what Connector/ODBC does) or set wait_timeout on the mysqld server so high that it in practice never times out.

  • You can also get these errors if you send a query to the server that is incorrect or too large. If mysqld receives a packet that is too large or out of order, it assumes that something has gone wrong with the client and closes the connection. If you need big queries (for example, if you are working with big BLOB columns), you can increase the query limit by setting the server's max_allowed_packet variable, which has a default value of 4MB. You may also need to increase the maximum packet size on the client end. More information on setting the packet size is given in Section B.5.2.10, “Packet Too Large”.

    An INSERT or REPLACE statement that inserts a great many rows can also cause these sorts of errors. Either one of these statements sends a single request to the server irrespective of the number of rows to be inserted; thus, you can often avoid the error by reducing the number of rows sent per INSERT or REPLACE.

  • You also get a lost connection if you are sending a packet 16MB or larger if your client is older than 4.0.8 and your server is 4.0.8 and above, or the other way around.

  • It is also possible to see this error if host name lookups fail (for example, if the DNS server on which your server or network relies goes down). This is because MySQL is dependent on the host system for name resolution, but has no way of knowing whether it is working—from MySQL's point of view the problem is indistinguishable from any other network timeout.

    You may also see the MySQL server has gone away error if MySQL is started with the --skip-networking option.

    Another networking issue that can cause this error occurs if the MySQL port (default 3306) is blocked by your firewall, thus preventing any connections at all to the MySQL server.

  • You can also encounter this error with applications that fork child processes, all of which try to use the same connection to the MySQL server. This can be avoided by using a separate connection for each child process.

  • You have encountered a bug where the server died while executing the query.

You can check whether the MySQL server died and restarted by executing mysqladmin version and examining the server's uptime. If the client connection was broken because mysqld crashed and restarted, you should concentrate on finding the reason for the crash. Start by checking whether issuing the query again kills the server again. See Section B.5.3.3, “What to Do If MySQL Keeps Crashing”.

You can get more information about the lost connections by starting mysqld with the log_error_verbosity system variable set to 3. This logs some of the disconnection messages in the hostname.err file. See Section 6.4.2, “The Error Log”.

If you want to create a bug report regarding this problem, be sure that you include the following information:

See also Section B.5.2.11, “Communication Errors and Aborted Connections”, and Section 1.7, “How to Report Bugs or Problems”.

B.5.2.10 Packet Too Large

A communication packet is a single SQL statement sent to the MySQL server, a single row that is sent to the client, or a binary log event sent from a master replication server to a slave.

The largest possible packet that can be transmitted to or from a MySQL 5.7 server or client is 1GB.

When a MySQL client or the mysqld server receives a packet bigger than max_allowed_packet bytes, it issues an ER_NET_PACKET_TOO_LARGE error and closes the connection. With some clients, you may also get a Lost connection to MySQL server during query error if the communication packet is too large.

Both the client and the server have their own max_allowed_packet variable, so if you want to handle big packets, you must increase this variable both in the client and in the server.

If you are using the mysql client program, its default max_allowed_packet variable is 16MB. To set a larger value, start mysql like this: 

shell> mysql --max_allowed_packet=32M

That sets the packet size to 32MB.

The server's default max_allowed_packet value is 4MB. You can increase this if the server needs to handle big queries (for example, if you are working with big BLOB columns). For example, to set the variable to 16MB, start the server like this: 

shell> mysqld --max_allowed_packet=16M

You can also use an option file to set max_allowed_packet. For example, to set the size for the server to 16MB, add the following lines in an option file: 

[mysqld]
max_allowed_packet=16M

It is safe to increase the value of this variable because the extra memory is allocated only when needed. For example, mysqld allocates more memory only when you issue a long query or when mysqld must return a large result row. The small default value of the variable is a precaution to catch incorrect packets between the client and server and also to ensure that you do not run out of memory by using large packets accidentally.

You can also get strange problems with large packets if you are using large BLOB values but have not given mysqld access to enough memory to handle the query. If you suspect this is the case, try adding ulimit -d 256000 to the beginning of the mysqld_safe script and restarting mysqld.

B.5.2.11 Communication Errors and Aborted Connections

If connection problems occur such as communication errors or aborted connections, use these sources of information to diagnose problems:

If you start the server with the log_error_verbosity system variable set to 3, you might find messages like this in your error log: 

[Note] Aborted connection 854 to db: 'employees' user: 'josh'

If a client is unable even to connect, the server increments the Aborted_connects status variable. Unsuccessful connection attempts can occur for the following reasons:

If these kinds of things happen, it might indicate that someone is trying to break into your server! If the general query log is enabled, messages for these types of problems are logged to it.

If a client successfully connects but later disconnects improperly or is terminated, the server increments the Aborted_clients status variable, and logs an Aborted connection message to the error log. The cause can be any of the following:

Other reasons for problems with aborted connections or aborted clients:

  • The max_allowed_packet variable value is too small or queries require more memory than you have allocated for mysqld. See Section B.5.2.10, “Packet Too Large”.

  • Use of Ethernet protocol with Linux, both half and full duplex. Some Linux Ethernet drivers have this bug. You should test for this bug by transferring a huge file using FTP between the client and server machines. If a transfer goes in burst-pause-burst-pause mode, you are experiencing a Linux duplex syndrome. Switch the duplex mode for both your network card and hub/switch to either full duplex or to half duplex and test the results to determine the best setting.

  • A problem with the thread library that causes interrupts on reads.

  • Badly configured TCP/IP.

  • Faulty Ethernets, hubs, switches, cables, and so forth. This can be diagnosed properly only by replacing hardware.

See also Section B.5.2.9, “MySQL server has gone away”.

B.5.2.12 The table is full

If a table-full error occurs, it may be that the disk is full or that the table has reached its maximum size. The effective maximum table size for MySQL databases is usually determined by operating system constraints on file sizes, not by MySQL internal limits. See Section C.10.3, “Limits on Table Size”.

B.5.2.13 Can't create/write to file

If you get an error of the following type for some queries, it means that MySQL cannot create a temporary file for the result set in the temporary directory: 

Can't create/write to file '\\sqla3fe_0.ism'.

The preceding error is a typical message for Windows; the Unix message is similar.

One fix is to start mysqld with the --tmpdir option or to add the option to the [mysqld] section of your option file. For example, to specify a directory of C:\temp, use these lines: 

[mysqld]
tmpdir=C:/temp

The C:\temp directory must exist and have sufficient space for the MySQL server to write to. See Section 5.2.6, “Using Option Files”.

Another cause of this error can be permissions issues. Make sure that the MySQL server can write to the tmpdir directory.

Check also the error code that you get with perror. One reason the server cannot write to a table is that the file system is full: 

shell> perror 28
OS error code  28:  No space left on device

If you get an error of the following type during startup, it indicates that the file system or directory used for storing data files is write protected. Provided that the write error is to a test file, the error is not serious and can be safely ignored. 

Can't create test file /usr/local/mysql/data/master.lower-test

B.5.2.14 Commands out of sync

If you get Commands out of sync; you can't run this command now in your client code, you are calling client functions in the wrong order.

This can happen, for example, if you are using mysql_use_result() and try to execute a new query before you have called mysql_free_result(). It can also happen if you try to execute two queries that return data without calling mysql_use_result() or mysql_store_result() in between.

B.5.2.15 Ignoring user

If you get the following error, it means that when mysqld was started or when it reloaded the grant tables, it found an account in the user table that had an invalid password.

Found wrong password for user 'some_user'@'some_host'; ignoring user

As a result, the account is simply ignored by the permission system.

The following list indicates possible causes of and fixes for this problem:

  • You may be running a new version of mysqld with an old user table. You can check this by executing mysqlshow mysql user to see whether the Password column is shorter than 16 characters. If so, you can correct this condition by running the scripts/add_long_password script.

  • The account has an old password (eight characters long). Update the account in the user table to have a new password.

  • You have specified a password in the user table without using the PASSWORD() function. Use mysql to update the account in the user table with a new password, making sure to use the PASSWORD() function: 

    mysql> UPDATE user SET Password=PASSWORD('new_password')
    -> WHERE User='some_user' AND Host='some_host';

B.5.2.16 Table 'tbl_name' doesn't exist

If you get either of the following errors, it usually means that no table exists in the default database with the given name: 

Table 'tbl_name' doesn't exist
Can't find file: 'tbl_name' (errno: 2)

In some cases, it may be that the table does exist but that you are referring to it incorrectly:

  • Because MySQL uses directories and files to store databases and tables, database and table names are case sensitive if they are located on a file system that has case-sensitive file names.

  • Even for file systems that are not case sensitive, such as on Windows, all references to a given table within a query must use the same lettercase.

You can check which tables are in the default database with SHOW TABLES. See Section 14.7.5, “SHOW Syntax”.

B.5.2.17 Can't initialize character set

You might see an error like this if you have character set problems: 

MySQL Connection Failed: Can't initialize character set charset_name

This error can have any of the following causes:

B.5.2.18 File Not Found and Similar Errors

If you get ERROR 'file_name' not found (errno: 23), Can't open file: file_name (errno: 24), or any other error with errno 23 or errno 24 from MySQL, it means that you have not allocated enough file descriptors for the MySQL server. You can use the perror utility to get a description of what the error number means: 

shell> perror 23
OS error code  23:  File table overflow
shell> perror 24
OS error code  24:  Too many open files
shell> perror 11
OS error code  11:  Resource temporarily unavailable

The problem here is that mysqld is trying to keep open too many files simultaneously. You can either tell mysqld not to open so many files at once or increase the number of file descriptors available to mysqld.

To tell mysqld to keep open fewer files at a time, you can make the table cache smaller by reducing the value of the table_open_cache system variable (the default value is 64). This may not entirely prevent running out of file descriptors because in some circumstances the server may attempt to extend the cache size temporarily, as described in Section 9.4.3.1, “How MySQL Opens and Closes Tables”. Reducing the value of max_connections also reduces the number of open files (the default value is 100).

To change the number of file descriptors available to mysqld, you can use the --open-files-limit option to mysqld_safe or set the open_files_limit system variable. See Section 6.1.4, “Server System Variables”. The easiest way to set these values is to add an option to your option file. See Section 5.2.6, “Using Option Files”. If you have an old version of mysqld that does not support setting the open files limit, you can edit the mysqld_safe script. There is a commented-out line ulimit -n 256 in the script. You can remove the “#” character to uncomment this line, and change the number 256 to set the number of file descriptors to be made available to mysqld.

--open-files-limit and ulimit can increase the number of file descriptors, but only up to the limit imposed by the operating system. There is also a “hard” limit that can be overridden only if you start mysqld_safe or mysqld as root (just remember that you also need to start the server with the --user option in this case so that it does not continue to run as root after it starts up). If you need to increase the operating system limit on the number of file descriptors available to each process, consult the documentation for your system.

Note

If you run the tcsh shell, ulimit does not work! tcsh also reports incorrect values when you ask for the current limits. In this case, you should start mysqld_safe using sh.

B.5.2.19 Table-Corruption Issues

If you have started mysqld with --myisam-recover-options, MySQL automatically checks and tries to repair MyISAM tables if they are marked as 'not closed properly' or 'crashed'. If this happens, MySQL writes an entry in the hostname.err file 'Warning: Checking table ...' which is followed by Warning: Repairing table if the table needs to be repaired. If you get a lot of these errors, without mysqld having died unexpectedly just before, then something is wrong and needs to be investigated further.

When the server detects MyISAM table corruption, it writes additional information to the error log, such as the name and line number of the source file, and the list of threads accessing the table. Example: Got an error from thread_id=1, mi_dynrec.c:368. This is useful information to include in bug reports.

See also Section 6.1.3, “Server Command Options”, and Section 26.5.1.7, “Making a Test Case If You Experience Table Corruption”.

B.5.3 Administration-Related Issues

B.5.3.1 Problems with File Permissions
B.5.3.2 How to Reset the Root Password
B.5.3.3 What to Do If MySQL Keeps Crashing
B.5.3.4 How MySQL Handles a Full Disk
B.5.3.5 Where MySQL Stores Temporary Files
B.5.3.6 How to Protect or Change the MySQL Unix Socket File
B.5.3.7 Time Zone Problems

B.5.3.1 Problems with File Permissions

If you have problems with file permissions, the UMASK or UMASK_DIR environment variable might be set incorrectly when mysqld starts. For example, MySQL might issue the following error message when you create a table: 

ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)

The default UMASK and UMASK_DIR values are 0640 and 0750, respectively (0660 and 0700 prior to MySQL 5.7.6). MySQL assumes that the value for UMASK or UMASK_DIR is in octal if it starts with a zero. For example, setting UMASK=0600 is equivalent to UMASK=384 because 0600 octal is 384 decimal.

To change the default UMASK value, start mysqld_safe as follows: 

shell> UMASK=384  # = 600 in octal
shell> export UMASK
shell> mysqld_safe &

By default, MySQL creates database directories with an access permission value of 0750. To modify this behavior, set the UMASK_DIR variable. If you set its value, new directories are created with the combined UMASK and UMASK_DIR values. For example, to give group access to all new directories, start mysqld_safe as follows: 

shell> UMASK_DIR=504  # = 770 in octal
shell> export UMASK_DIR
shell> mysqld_safe &

For additional details, see Section 2.12, “Environment Variables”.

B.5.3.2 How to Reset the Root Password

If you have never assigned a root password for MySQL, the server does not require a password at all for connecting as root. However, this is insecure. For instructions on assigning a password, see Section 2.10.4, “Securing the Initial MySQL Accounts”.

If you know the root password and want to change it, see Section 14.7.1.1, “ALTER USER Syntax”, and Section 14.7.1.7, “SET PASSWORD Syntax”.

If you assigned a root password previously but have forgotten it, you can assign a new password. The following sections provide instructions for Windows and Unix and Unix-like systems, as well as generic instructions that apply to any system.

B.5.3.2.1 Resetting the Root Password: Windows Systems

On Windows, use the following procedure to reset the password for the MySQL 'root'@'localhost' account. To change the password for a root account with a different host name part, modify the instructions to use that host name.

  1. Log on to your system as Administrator.

  2. Stop the MySQL server if it is running. For a server that is running as a Windows service, go to the Services manager: From the Start menu, select Control Panel, then Administrative Tools, then Services. Find the MySQL service in the list and stop it.

    If your server is not running as a service, you may need to use the Task Manager to force it to stop.

  3. Create a text file containing the password-assignment statement on a single line. Replace the password with the password that you want to use.

    MySQL 5.7.6 and later: 

    ALTER USER 'root'@'localhost' IDENTIFIED BY 'MyNewPass';

    MySQL 5.7.5 and earlier: 

    SET PASSWORD FOR 'root'@'localhost' = PASSWORD('MyNewPass');

  4. Save the file. This example assumes that you name the file C:\mysql-init.txt.

  5. Open a console window to get to the command prompt: From the Start menu, select Run, then enter cmd as the command to be run.

  6. Start the MySQL server with the special --init-file option (notice that the backslash in the option value is doubled): 

    C:\> cd "C:\Program Files\MySQL\MySQL Server 5.7\bin"
C:\> mysqld --init-file=C:\\mysql-init.txt

    If you installed MySQL to a different location, adjust the cd command accordingly.

    The server executes the contents of the file named by the --init-file option at startup, changing the 'root'@'localhost' account password.

    To have server output to appear in the console window rather than in a log file, add the --console option to the mysqld command.

    If you installed MySQL using the MySQL Installation Wizard, you may need to specify a --defaults-file option. For example: 

    C:\> mysqld
         --defaults-file="C:\\ProgramData\\MySQL\\MySQL Server 5.7\\my.ini"
         --init-file=C:\\mysql-init.txt

    The appropriate --defaults-file setting can be found using the Services Manager: From the Start menu, select Control Panel, then Administrative Tools, then Services. Find the MySQL service in the list, right-click it, and choose the Properties option. The Path to executable field contains the --defaults-file setting.

  7. After the server has started successfully, delete C:\mysql-init.txt.

You should now be able to connect to the MySQL server as root using the new password. Stop the MySQL server and restart it normally. If you run the server as a service, start it from the Windows Services window. If you start the server manually, use whatever command you normally use.

If the ALTER USER statement fails to reset the password, try repeating the procedure using the following statements to modify the user table directly: 

UPDATE mysql.user 
    SET authentication_string = PASSWORD('MyNewPass'), password_expired = 'N'
    WHERE User = 'root' AND Host = 'localhost';
FLUSH PRIVILEGES;
B.5.3.2.2 Resetting the Root Password: Unix and Unix-Like Systems

On Unix, use the following procedure to reset the password for the MySQL 'root'@'localhost' account. To change the password for a root account with a different host name part, modify the instructions to use that host name.

The instructions assume that you will start the MySQL server from the Unix login account that you normally use for running it. For example, if you run the server using the mysql login account, you should log in as mysql before using the instructions. Alternatively, you can log in as root, but in this case you must start mysqld with the --user=mysql option. If you start the server as root without using --user=mysql, the server may create root-owned files in the data directory, such as log files, and these may cause permission-related problems for future server startups. If that happens, you will need to either change the ownership of the files to mysql or remove them.

  1. Log on to your system as the Unix user that the MySQL server runs as (for example, mysql).

  2. Stop the MySQL server if it is running. Locate the .pid file that contains the server's process ID. The exact location and name of this file depend on your distribution, host name, and configuration. Common locations are /var/lib/mysql/, /var/run/mysqld/, and /usr/local/mysql/data/. Generally, the file name has an extension of .pid and begins with either mysqld or your system's host name.

    Stop the MySQL server by sending a normal kill (not kill -9) to the mysqld process. Use the actual path name of the .pid file in the following command: 

    shell> kill `cat /mysql-data-directory/host_name.pid`

    Use backticks (not forward quotation marks) with the cat command. These cause the output of cat to be substituted into the kill command.

  3. Create a text file containing the password-assignment statement on a single line. Replace the password with the password that you want to use.

    MySQL 5.7.6 and later: 

    ALTER USER 'root'@'localhost' IDENTIFIED BY 'MyNewPass';

    MySQL 5.7.5 and earlier: 

    SET PASSWORD FOR 'root'@'localhost' = PASSWORD('MyNewPass');

  4. Save the file. This example assumes that you name the file /home/me/mysql-init. The file contains the password, so do not save it where it can be read by other users. If you are not logged in as mysql (the user the server runs as), make sure that the file has permissions that permit mysql to read it.

  5. Start the MySQL server with the special --init-file option: 

    shell> mysqld_safe --init-file=/home/me/mysql-init &

    The server executes the contents of the file named by the --init-file option at startup, changing the 'root'@'localhost' account password.

  6. After the server has started successfully, delete /home/me/mysql-init.

You should now be able to connect to the MySQL server as root using the new password. Stop the server and restart it normally.

If the ALTER USER statement fails to reset the password, try repeating the procedure using the following statements to modify the user table directly: 

UPDATE mysql.user 
    SET authentication_string = PASSWORD('MyNewPass'), password_expired = 'N'
    WHERE User = 'root' AND Host = 'localhost';
FLUSH PRIVILEGES;
B.5.3.2.3 Resetting the Root Password: Generic Instructions

The preceding sections provide password-resetting instructions specifically for Windows and Unix and Unix-like systems. Alternatively, on any platform, you can reset the password using the mysql client (but this approach is less secure):

  1. Stop the MySQL server if necessary, then restart it with the --skip-grant-tables option. This enables anyone to connect without a password and with all privileges, and disables account-management statements such as ALTER USER and SET PASSWORD. Because this is insecure, you might want to use --skip-grant-tables in conjunction with --skip-networking to prevent remote clients from connecting.

  2. Connect to the MySQL server using the mysql client; no password is necessary because the server was started with --skip-grant-tables: 

    shell> mysql

  3. In the mysql client, tell the server to reload the grant tables so that account-management statements work: 

    mysql> FLUSH PRIVILEGES;

    Then change the 'root'@'localhost' account password. Replace the password with the password that you want to use. To change the password for a root account with a different host name part, modify the instructions to use that host name.

    MySQL 5.7.6 and later: 

    mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY 'MyNewPass';

    MySQL 5.7.5 and earlier: 

    mysql> SET PASSWORD FOR 'root'@'localhost' = PASSWORD('MyNewPass');

You should now be able to connect to the MySQL server as root using the new password. Stop the server and restart it normally (without the --skip-grant-tables and --skip-networking options).

If the ALTER USER statement fails to reset the password, try repeating the procedure using the following statements to modify the user table directly: 

UPDATE mysql.user SET authentication_string = PASSWORD('MyNewPass')
WHERE User = 'root' AND Host = 'localhost';
FLUSH PRIVILEGES;

B.5.3.3 What to Do If MySQL Keeps Crashing

Each MySQL version is tested on many platforms before it is released. This does not mean that there are no bugs in MySQL, but if there are bugs, they should be very few and can be hard to find. If you have a problem, it always helps if you try to find out exactly what crashes your system, because you have a much better chance of getting the problem fixed quickly.

First, you should try to find out whether the problem is that the mysqld server dies or whether your problem has to do with your client. You can check how long your mysqld server has been up by executing mysqladmin version. If mysqld has died and restarted, you may find the reason by looking in the server's error log. See Section 6.4.2, “The Error Log”.

On some systems, you can find in the error log a stack trace of where mysqld died that you can resolve with the resolve_stack_dump program. See Section 26.5, “Debugging and Porting MySQL”. Note that the variable values written in the error log may not always be 100% correct.

Many server crashes are caused by corrupted data files or index files. MySQL updates the files on disk with the write() system call after every SQL statement and before the client is notified about the result. (This is not true if you are running with --delay-key-write, in which case data files are written but not index files.) This means that data file contents are safe even if mysqld crashes, because the operating system ensures that the unflushed data is written to disk. You can force MySQL to flush everything to disk after every SQL statement by starting mysqld with the --flush option.

The preceding means that normally you should not get corrupted tables unless one of the following happens:

  • The MySQL server or the server host was killed in the middle of an update.

  • You have found a bug in mysqld that caused it to die in the middle of an update.

  • Some external program is manipulating data files or index files at the same time as mysqld without locking the table properly.

  • You are running many mysqld servers using the same data directory on a system that does not support good file system locks (normally handled by the lockd lock manager), or you are running multiple servers with external locking disabled.

  • You have a crashed data file or index file that contains very corrupt data that confused mysqld.

  • You have found a bug in the data storage code. This isn't likely, but it is at least possible. In this case, you can try to change the storage engine to another engine by using ALTER TABLE on a repaired copy of the table.

Because it is very difficult to know why something is crashing, first try to check whether things that work for others crash for you. Try the following things:

  • Stop the mysqld server with mysqladmin shutdown, run myisamchk --silent --force */*.MYI from the data directory to check all MyISAM tables, and restart mysqld. This ensures that you are running from a clean state. See Chapter 6, MySQL Server Administration.

  • Start mysqld with the general query log enabled (see Section 6.4.3, “The General Query Log”). Then try to determine from the information written to the log whether some specific query kills the server. About 95% of all bugs are related to a particular query. Normally, this is one of the last queries in the log file just before the server restarts. See Section 6.4.3, “The General Query Log”. If you can repeatedly kill MySQL with a specific query, even when you have checked all tables just before issuing it, then you have isolated the bug and should submit a bug report for it. See Section 1.7, “How to Report Bugs or Problems”.

  • Try to make a test case that we can use to repeat the problem. See Section 26.5, “Debugging and Porting MySQL”.

  • Try the fork_big.pl script. (It is located in the tests directory of source distributions.)

  • Configuring MySQL for debugging makes it much easier to gather information about possible errors if something goes wrong. Reconfigure MySQL with the -DWITH_DEBUG=1 option to CMake and then recompile. See Section 26.5, “Debugging and Porting MySQL”.

  • Make sure that you have applied the latest patches for your operating system.

  • Use the --skip-external-locking option to mysqld. On some systems, the lockd lock manager does not work properly; the --skip-external-locking option tells mysqld not to use external locking. (This means that you cannot run two mysqld servers on the same data directory and that you must be careful if you use myisamchk. Nevertheless, it may be instructive to try the option as a test.)

  • If mysqld appears to be running but not responding, try mysqladmin -u root processlist. Sometimes mysqld is not hung even though it seems unresponsive. The problem may be that all connections are in use, or there may be some internal lock problem. mysqladmin -u root processlist usually is able to make a connection even in these cases, and can provide useful information about the current number of connections and their status.

  • Run the command mysqladmin -i 5 status or mysqladmin -i 5 -r status in a separate window to produce statistics while running other queries.

  • Try the following:

    1. Start mysqld from gdb (or another debugger). See Section 26.5, “Debugging and Porting MySQL”.

    2. Run your test scripts.

    3. Print the backtrace and the local variables at the three lowest levels. In gdb, you can do this with the following commands when mysqld has crashed inside gdb: 

      backtrace
info local
up
info local
up
info local

      With gdb, you can also examine which threads exist with info threads and switch to a specific thread with thread N, where N is the thread ID.

  • Try to simulate your application with a Perl script to force MySQL to crash or misbehave.

  • Send a normal bug report. See Section 1.7, “How to Report Bugs or Problems”. Be even more detailed than usual. Because MySQL works for many people, the crash might result from something that exists only on your computer (for example, an error that is related to your particular system libraries).

  • If you have a problem with tables containing dynamic-length rows and you are using only VARCHAR columns (not BLOB or TEXT columns), you can try to change all VARCHAR to CHAR with ALTER TABLE. This forces MySQL to use fixed-size rows. Fixed-size rows take a little extra space, but are much more tolerant to corruption.

    The current dynamic row code has been in use for several years with very few problems, but dynamic-length rows are by nature more prone to errors, so it may be a good idea to try this strategy to see whether it helps.

  • Consider the possibility of hardware faults when diagnosing problems. Defective hardware can be the cause of data corruption. Pay particular attention to your memory and disk subsystems when troubleshooting hardware.

B.5.3.4 How MySQL Handles a Full Disk

This section describes how MySQL responds to disk-full errors (such as “no space left on device”), and to quota-exceeded errors (such as “write failed” or “user block limit reached”).

This section is relevant for writes to MyISAM tables. It also applies for writes to binary log files and binary log index file, except that references to “row” and “record” should be understood to mean “event.”

When a disk-full condition occurs, MySQL does the following:

  • It checks once every minute to see whether there is enough space to write the current row. If there is enough space, it continues as if nothing had happened.

  • Every 10 minutes it writes an entry to the log file, warning about the disk-full condition.

To alleviate the problem, take the following actions:

  • To continue, you only have to free enough disk space to insert all records.

  • Alternatively, to abort the thread, use mysqladmin kill. The thread is aborted the next time it checks the disk (in one minute).

  • Other threads might be waiting for the table that caused the disk-full condition. If you have several “locked” threads, killing the one thread that is waiting on the disk-full condition enables the other threads to continue.

Exceptions to the preceding behavior are when you use REPAIR TABLE or OPTIMIZE TABLE or when the indexes are created in a batch after LOAD DATA INFILE or after an ALTER TABLE statement. All of these statements may create large temporary files that, if left to themselves, would cause big problems for the rest of the system. If the disk becomes full while MySQL is doing any of these operations, it removes the big temporary files and mark the table as crashed. The exception is that for ALTER TABLE, the old table is left unchanged.

B.5.3.5 Where MySQL Stores Temporary Files

On Unix, MySQL uses the value of the TMPDIR environment variable as the path name of the directory in which to store temporary files. If TMPDIR is not set, MySQL uses the system default, which is usually /tmp, /var/tmp, or /usr/tmp.

On Windows, MySQL checks in order the values of the TMPDIR, TEMP, and TMP environment variables. For the first one found to be set, MySQL uses it and does not check those remaining. If none of TMPDIR, TEMP, or TMP are set, MySQL uses the Windows system default, which is usually C:\windows\temp\.

If the file system containing your temporary file directory is too small, you can use the mysqld --tmpdir option to specify a directory in a file system where you have enough space. On replication slaves, you can use --slave-load-tmpdir to specify a separate directory for holding temporary files when replicating LOAD DATA INFILE statements.

The --tmpdir option can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (“:”) on Unix and semicolon characters (“;”) on Windows.

Note

To spread the load effectively, these paths should be located on different physical disks, not different partitions of the same disk.

If the MySQL server is acting as a replication slave, you should be sure to set --slave-load-tmpdir not to point to a directory that is on a memory-based file system or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the slave temporary file directory are lost when the server restarts, replication fails.

MySQL arranges that temporary files are removed if mysqld is terminated. On platforms that support it (such as Unix), this is done by unlinking the file after opening it. The disadvantage of this is that the name does not appear in directory listings and you do not see a big temporary file that fills up the file system in which the temporary file directory is located. (In such cases, lsof +L1 may be helpful in identifying large files associated with mysqld.)

When sorting (ORDER BY or GROUP BY), MySQL normally uses one or two temporary files. The maximum disk space required is determined by the following expression: 

(length of what is sorted + sizeof(row pointer))
* number of matched rows
* 2

The row pointer size is usually four bytes, but may grow in the future for really big tables.

For some SELECT queries, MySQL also creates temporary SQL tables. These are not hidden and have names of the form SQL_*.

In most cases, ALTER TABLE creates a temporary copy of the original table in the same directory as the original table.

Where InnoDB Stores Temporary Files

As of MySQL 5.7.1, non-compressed InnoDB temporary tables are stored in a temporary tablespace file named ibtmp1, which is located in the MySQL data directory (datadir) by default. The innodb_temp_data_file_path option can be used at startup to specify a different file name and location. Compressed InnoDB temporary tables are stored in their own independent tablespace files (.ibd files) in the path specified by the TMPDIR environment variable.

If an ALTER TABLE operation on an InnoDB table uses the ALGORITHM=COPY technique, InnoDB creates a temporary copy of the table in the same directory as the original table. Temporary table file names begin with an #sql- prefix and only appear briefly during the ALTER TABLE operation.

If an ALTER TABLE operation rebuilds an InnoDB table using the ALGORITHM=INPLACE technique (online DDL), InnoDB creates an intermediate copy of the table the same directory as the original table. Intermediate table file names begin with an #sql-ib prefix and only appear briefly during the ALTER TABLE operation.

ALTER TABLE operations that rebuild an InnoDB table using the ALGORITHM=INPLACE technique (online DDL) also create temporary sort files in the MySQL temporary directory ($TMPDIR on Unix, %TEMP% on Windows, or the directory specified by the --tmpdir configuration option). If the temporary directory is not large enough to hold such files, you may need to reconfigure tmpdir. Alternatively, you can define a separate temporary directory for InnoDB online ALTER TABLE operations using the innodb_tmpdir configuration option. This option was introduced in MySQL 5.7.11 to help avoid temporary directory overflows that could occur as a result of large temporary sort files created during online ALTER TABLE operations. innodb_tmpdir can be configured dynamically using a SET GLOBAL or SET SESSION statement.

The innodb_tmpdir option is not applicable to intermediate table files, which are always created in the same directory as the original table.

In replication environments, only consider replicating an innodb_tmpdir setting if all servers have the same operating system environment. Otherwise, replicating an innodb_tmpdir setting could result in a replication failure when running online ALTER TABLE operations. If server operating environments differ, it is recommended that you configure innodb_tmpdir on each server individually.

For more information about online DDL, Section 15.11, “InnoDB and Online DDL”.

B.5.3.6 How to Protect or Change the MySQL Unix Socket File

The default location for the Unix socket file that the server uses for communication with local clients is /tmp/mysql.sock. (For some distribution formats, the directory might be different, such as /var/lib/mysql for RPMs.)

On some versions of Unix, anyone can delete files in the /tmp directory or other similar directories used for temporary files. If the socket file is located in such a directory on your system, this might cause problems.

On most versions of Unix, you can protect your /tmp directory so that files can be deleted only by their owners or the superuser (root). To do this, set the sticky bit on the /tmp directory by logging in as root and using the following command: 

shell> chmod +t /tmp

You can check whether the sticky bit is set by executing ls -ld /tmp. If the last permission character is t, the bit is set.

Another approach is to change the place where the server creates the Unix socket file. If you do this, you should also let client programs know the new location of the file. You can specify the file location in several ways:

You can test whether the new socket location works by attempting to connect to the server with this command: 

shell> mysqladmin --socket=/path/to/socket version

B.5.3.7 Time Zone Problems

If you have a problem with SELECT NOW() returning values in UTC and not your local time, you have to tell the server your current time zone. The same applies if UNIX_TIMESTAMP() returns the wrong value. This should be done for the environment in which the server runs; for example, in mysqld_safe or mysql.server. See Section 2.12, “Environment Variables”.

You can set the time zone for the server with the --timezone=timezone_name option to mysqld_safe. You can also set it by setting the TZ environment variable before you start mysqld.

The permissible values for --timezone or TZ are system dependent. Consult your operating system documentation to see what values are acceptable.

B.5.4 Query-Related Issues

B.5.4.1 Case Sensitivity in String Searches
B.5.4.2 Problems Using DATE Columns
B.5.4.3 Problems with NULL Values
B.5.4.4 Problems with Column Aliases
B.5.4.5 Rollback Failure for Nontransactional Tables
B.5.4.6 Deleting Rows from Related Tables
B.5.4.7 Solving Problems with No Matching Rows
B.5.4.8 Problems with Floating-Point Values

B.5.4.1 Case Sensitivity in String Searches

For nonbinary strings (CHAR, VARCHAR, TEXT), string searches use the collation of the comparison operands. For binary strings (BINARY, VARBINARY, BLOB), comparisons use the numeric values of the bytes in the operands; this means that for alphabetic characters, comparisons will be case sensitive.

A comparison between a nonbinary string and binary string is treated as a comparison of binary strings.

Simple comparison operations (>=, >, =, <, <=, sorting, and grouping) are based on each character's “sort value.” Characters with the same sort value are treated as the same character. For example, if “e” and “é” have the same sort value in a given collation, they compare as equal.

The default character set and collation are latin1 and latin1_swedish_ci, so nonbinary string comparisons are case insensitive by default. This means that if you search with col_name LIKE 'a%', you get all column values that start with A or a. To make this search case sensitive, make sure that one of the operands has a case sensitive or binary collation. For example, if you are comparing a column and a string that both have the latin1 character set, you can use the COLLATE operator to cause either operand to have the latin1_general_cs or latin1_bin collation: 

col_name COLLATE latin1_general_cs LIKE 'a%'
col_name LIKE 'a%' COLLATE latin1_general_cs
col_name COLLATE latin1_bin LIKE 'a%'
col_name LIKE 'a%' COLLATE latin1_bin

If you want a column always to be treated in case-sensitive fashion, declare it with a case sensitive or binary collation. See Section 14.1.18, “CREATE TABLE Syntax”.

To cause a case-sensitive comparison of nonbinary strings to be case insensitive, use COLLATE to name a case-insensitive collation. The strings in the following example normally are case sensitive, but COLLATE changes the comparison to be case insensitive: 

mysql> SET @s1 = 'MySQL' COLLATE latin1_bin,
    ->     @s2 = 'mysql' COLLATE latin1_bin;
mysql> SELECT @s1 = @s2;
+-----------+
| @s1 = @s2 |
+-----------+
|         0 |
+-----------+
mysql> SELECT @s1 COLLATE latin1_swedish_ci = @s2;
+-------------------------------------+
| @s1 COLLATE latin1_swedish_ci = @s2 |
+-------------------------------------+
|                                   1 |
+-------------------------------------+

A binary string is case sensitive in comparisons. To compare the string as case insensitive, convert it to a nonbinary string and use COLLATE to name a case-insensitive collation: 

mysql> SET @s = BINARY 'MySQL';
mysql> SELECT @s = 'mysql';
+--------------+
| @s = 'mysql' |
+--------------+
|            0 |
+--------------+
mysql> SELECT CONVERT(@s USING latin1) COLLATE latin1_swedish_ci = 'mysql';
+--------------------------------------------------------------+
| CONVERT(@s USING latin1) COLLATE latin1_swedish_ci = 'mysql' |
+--------------------------------------------------------------+
|                                                            1 |
+--------------------------------------------------------------+

To determine whether a value will compare as a nonbinary or binary string, use the COLLATION() function. This example shows that VERSION() returns a string that has a case-insensitive collation, so comparisons are case insensitive: 

mysql> SELECT COLLATION(VERSION());
+----------------------+
| COLLATION(VERSION()) |
+----------------------+
| utf8_general_ci      |
+----------------------+

For binary strings, the collation value is binary, so comparisons will be case sensitive. One context in which you will see binary is for compression and encryption functions, which return binary strings as a general rule: string: 

mysql> SELECT COLLATION(ENCRYPT('x')), COLLATION(SHA1('x'));
+-------------------------+----------------------+
| COLLATION(ENCRYPT('x')) | COLLATION(SHA1('x')) |
+-------------------------+----------------------+
| binary                  | binary               |
+-------------------------+----------------------+

To check the sort value of a string, the WEIGHT_STRING() may be helpful. See Section 13.5, “String Functions”.

B.5.4.2 Problems Using DATE Columns

The format of a DATE value is 'YYYY-MM-DD'. According to standard SQL, no other format is permitted. You should use this format in UPDATE expressions and in the WHERE clause of SELECT statements. For example: 

SELECT * FROM t1 WHERE date >= '2003-05-05';

As a convenience, MySQL automatically converts a date to a number if the date is used in a numeric context and vice versa. MySQL also permits a “relaxed” string format when updating and in a WHERE clause that compares a date to a DATE, DATETIME, or TIMESTAMP column. “Relaxed” format means that any punctuation character may be used as the separator between parts. For example, '2004-08-15' and '2004#08#15' are equivalent. MySQL can also convert a string containing no separators (such as '20040815'), provided it makes sense as a date.

When you compare a DATE, TIME, DATETIME, or TIMESTAMP to a constant string with the <, <=, =, >=, >, or BETWEEN operators, MySQL normally converts the string to an internal long integer for faster comparison (and also for a bit more “relaxed” string checking). However, this conversion is subject to the following exceptions:

  • When you compare two columns

  • When you compare a DATE, TIME, DATETIME, or TIMESTAMP column to an expression

  • When you use any comparison method other than those just listed, such as IN or STRCMP().

For those exceptions, the comparison is done by converting the objects to strings and performing a string comparison.

To be on the safe side, assume that strings are compared as strings and use the appropriate string functions if you want to compare a temporal value to a string.

The special “zero” date '0000-00-00' can be stored and retrieved as '0000-00-00'. When a '0000-00-00' date is used through Connector/ODBC, it is automatically converted to NULL because ODBC cannot handle that kind of date.

Because MySQL performs the conversions just described, the following statements work (assume that idate is a DATE column): 

INSERT INTO t1 (idate) VALUES (19970505);
INSERT INTO t1 (idate) VALUES ('19970505');
INSERT INTO t1 (idate) VALUES ('97-05-05');
INSERT INTO t1 (idate) VALUES ('1997.05.05');
INSERT INTO t1 (idate) VALUES ('1997 05 05');
INSERT INTO t1 (idate) VALUES ('0000-00-00');

SELECT idate FROM t1 WHERE idate >= '1997-05-05';
SELECT idate FROM t1 WHERE idate >= 19970505;
SELECT MOD(idate,100) FROM t1 WHERE idate >= 19970505;
SELECT idate FROM t1 WHERE idate >= '19970505';

However, the following statement does not work: 

SELECT idate FROM t1 WHERE STRCMP(idate,'20030505')=0;

STRCMP() is a string function, so it converts idate to a string in 'YYYY-MM-DD' format and performs a string comparison. It does not convert '20030505' to the date '2003-05-05' and perform a date comparison.

If you enable the ALLOW_INVALID_DATES SQL mode, MySQL permits you to store dates that are given only limited checking: MySQL requires only that the day is in the range from 1 to 31 and the month is in the range from 1 to 12. This makes MySQL very convenient for Web applications where you obtain year, month, and day in three different fields and you want to store exactly what the user inserted (without date validation).

MySQL permits you to store dates where the day or month and day are zero. This is convenient if you want to store a birthdate in a DATE column and you know only part of the date. To disallow zero month or day parts in dates, enable the NO_ZERO_IN_DATE mode.

MySQL permits you to store a “zero” value of '0000-00-00' as a “dummy date.” This is in some cases more convenient than using NULL values. If a date to be stored in a DATE column cannot be converted to any reasonable value, MySQL stores '0000-00-00'. To disallow '0000-00-00', enable the NO_ZERO_DATE mode.

To have MySQL check all dates and accept only legal dates (unless overridden by IGNORE), set the sql_mode system variable to "NO_ZERO_IN_DATE,NO_ZERO_DATE".

B.5.4.3 Problems with NULL Values

The concept of the NULL value is a common source of confusion for newcomers to SQL, who often think that NULL is the same thing as an empty string ''. This is not the case. For example, the following statements are completely different: 

mysql> INSERT INTO my_table (phone) VALUES (NULL);
mysql> INSERT INTO my_table (phone) VALUES ('');

Both statements insert a value into the phone column, but the first inserts a NULL value and the second inserts an empty string. The meaning of the first can be regarded as “phone number is not known” and the meaning of the second can be regarded as “the person is known to have no phone, and thus no phone number.”

To help with NULL handling, you can use the IS NULL and IS NOT NULL operators and the IFNULL() function.

In SQL, the NULL value is never true in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression. All columns in the following example return NULL: 

mysql> SELECT NULL, 1+NULL, CONCAT('Invisible',NULL);

To search for column values that are NULL, you cannot use an expr = NULL test. The following statement returns no rows, because expr = NULL is never true for any expression: 

mysql> SELECT * FROM my_table WHERE phone = NULL;

To look for NULL values, you must use the IS NULL test. The following statements show how to find the NULL phone number and the empty phone number: 

mysql> SELECT * FROM my_table WHERE phone IS NULL;
mysql> SELECT * FROM my_table WHERE phone = '';

See Section 4.3.4.6, “Working with NULL Values”, for additional information and examples.

You can add an index on a column that can have NULL values if you are using the MyISAM, InnoDB, or MEMORY storage engine. Otherwise, you must declare an indexed column NOT NULL, and you cannot insert NULL into the column.

When reading data with LOAD DATA INFILE, empty or missing columns are updated with ''. To load a NULL value into a column, use \N in the data file. The literal word “NULL” may also be used under some circumstances. See Section 14.2.6, “LOAD DATA INFILE Syntax”.

When using DISTINCT, GROUP BY, or ORDER BY, all NULL values are regarded as equal.

When using ORDER BY, NULL values are presented first, or last if you specify DESC to sort in descending order.

Aggregate (summary) functions such as COUNT(), MIN(), and SUM() ignore NULL values. The exception to this is COUNT(*), which counts rows and not individual column values. For example, the following statement produces two counts. The first is a count of the number of rows in the table, and the second is a count of the number of non-NULL values in the age column: 

mysql> SELECT COUNT(*), COUNT(age) FROM person;

For some data types, MySQL handles NULL values specially. If you insert NULL into a TIMESTAMP column, the current date and time is inserted. If you insert NULL into an integer or floating-point column that has the AUTO_INCREMENT attribute, the next number in the sequence is inserted.

B.5.4.4 Problems with Column Aliases

An alias can be used in a query select list to give a column a different name. You can use the alias in GROUP BY, ORDER BY, or HAVING clauses to refer to the column: 

SELECT SQRT(a*b) AS root FROM tbl_name
  GROUP BY root HAVING root > 0;
SELECT id, COUNT(*) AS cnt FROM tbl_name
  GROUP BY id HAVING cnt > 0;
SELECT id AS 'Customer identity' FROM tbl_name;

Standard SQL disallows references to column aliases in a WHERE clause. This restriction is imposed because when the WHERE clause is evaluated, the column value may not yet have been determined. For example, the following query is illegal: 

SELECT id, COUNT(*) AS cnt FROM tbl_name
  WHERE cnt > 0 GROUP BY id;

The WHERE clause determines which rows should be included in the GROUP BY clause, but it refers to the alias of a column value that is not known until after the rows have been selected, and grouped by the GROUP BY.

In the select list of a query, a quoted column alias can be specified using identifier or string quoting characters: 

SELECT 1 AS `one`, 2 AS 'two';

Elsewhere in the statement, quoted references to the alias must use identifier quoting or the reference is treated as a string literal. For example, this statement groups by the values in column id, referenced using the alias `a`: 

SELECT id AS 'a', COUNT(*) AS cnt FROM tbl_name
  GROUP BY `a`;

But this statement groups by the literal string 'a' and will not work as expected: 

SELECT id AS 'a', COUNT(*) AS cnt FROM tbl_name
  GROUP BY 'a';

B.5.4.5 Rollback Failure for Nontransactional Tables

If you receive the following message when trying to perform a ROLLBACK, it means that one or more of the tables you used in the transaction do not support transactions: 

Warning: Some non-transactional changed tables couldn't be rolled back

These nontransactional tables are not affected by the ROLLBACK statement.

If you were not deliberately mixing transactional and nontransactional tables within the transaction, the most likely cause for this message is that a table you thought was transactional actually is not. This can happen if you try to create a table using a transactional storage engine that is not supported by your mysqld server (or that was disabled with a startup option). If mysqld does not support a storage engine, it instead creates the table as a MyISAM table, which is nontransactional.

You can check the storage engine for a table by using either of these statements: 

SHOW TABLE STATUS LIKE 'tbl_name';
SHOW CREATE TABLE tbl_name;

See Section 14.7.5.36, “SHOW TABLE STATUS Syntax”, and Section 14.7.5.10, “SHOW CREATE TABLE Syntax”.

To check which storage engines your mysqld server supports, use this statement: 

SHOW ENGINES;

See Section 14.7.5.16, “SHOW ENGINES Syntax” for full details.

B.5.4.6 Deleting Rows from Related Tables

If the total length of the DELETE statement for related_table is more than 1MB (the default value of the max_allowed_packet system variable), you should split it into smaller parts and execute multiple DELETE statements. You probably get the fastest DELETE by specifying only 100 to 1,000 related_column values per statement if the related_column is indexed. If the related_column isn't indexed, the speed is independent of the number of arguments in the IN clause.

B.5.4.7 Solving Problems with No Matching Rows

If you have a complicated query that uses many tables but that returns no rows, you should use the following procedure to find out what is wrong:

  1. Test the query with EXPLAIN to check whether you can find something that is obviously wrong. See Section 14.8.2, “EXPLAIN Syntax”.

  2. Select only those columns that are used in the WHERE clause.

  3. Remove one table at a time from the query until it returns some rows. If the tables are large, it is a good idea to use LIMIT 10 with the query.

  4. Issue a SELECT for the column that should have matched a row against the table that was last removed from the query.

  5. If you are comparing FLOAT or DOUBLE columns with numbers that have decimals, you cannot use equality (=) comparisons. This problem is common in most computer languages because not all floating-point values can be stored with exact precision. In some cases, changing the FLOAT to a DOUBLE fixes this. See Section B.5.4.8, “Problems with Floating-Point Values”.

  6. If you still cannot figure out what is wrong, create a minimal test that can be run with mysql test < query.sql that shows your problems. You can create a test file by dumping the tables with mysqldump --quick db_name tbl_name_1 ... tbl_name_n > query.sql. Open the file in an editor, remove some insert lines (if there are more than needed to demonstrate the problem), and add your SELECT statement at the end of the file.

    Verify that the test file demonstrates the problem by executing these commands: 

    shell> mysqladmin create test2
shell> mysql test2 < query.sql

    Attach the test file to a bug report, which you can file using the instructions in Section 1.7, “How to Report Bugs or Problems”.

B.5.4.8 Problems with Floating-Point Values

Floating-point numbers sometimes cause confusion because they are approximate and not stored as exact values. A floating-point value as written in an SQL statement may not be the same as the value represented internally. Attempts to treat floating-point values as exact in comparisons may lead to problems. They are also subject to platform or implementation dependencies. The FLOAT and DOUBLE data types are subject to these issues. For DECIMAL columns, MySQL performs operations with a precision of 65 decimal digits, which should solve most common inaccuracy problems.

The following example uses DOUBLE to demonstrate how calculations that are done using floating-point operations are subject to floating-point error. 

mysql> CREATE TABLE t1 (i INT, d1 DOUBLE, d2 DOUBLE);
mysql> INSERT INTO t1 VALUES (1, 101.40, 21.40), (1, -80.00, 0.00),
    -> (2, 0.00, 0.00), (2, -13.20, 0.00), (2, 59.60, 46.40),
    -> (2, 30.40, 30.40), (3, 37.00, 7.40), (3, -29.60, 0.00),
    -> (4, 60.00, 15.40), (4, -10.60, 0.00), (4, -34.00, 0.00),
    -> (5, 33.00, 0.00), (5, -25.80, 0.00), (5, 0.00, 7.20),
    -> (6, 0.00, 0.00), (6, -51.40, 0.00);

mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b
    -> FROM t1 GROUP BY i HAVING a <> b;

+------+-------+------+
| i    | a     | b    |
+------+-------+------+
|    1 |  21.4 | 21.4 |
|    2 |  76.8 | 76.8 |
|    3 |   7.4 |  7.4 |
|    4 |  15.4 | 15.4 |
|    5 |   7.2 |  7.2 |
|    6 | -51.4 |    0 |
+------+-------+------+

The result is correct. Although the first five records look like they should not satisfy the comparison (the values of a and b do not appear to be different), they may do so because the difference between the numbers shows up around the tenth decimal or so, depending on factors such as computer architecture or the compiler version or optimization level. For example, different CPUs may evaluate floating-point numbers differently.

If columns d1 and d2 had been defined as DECIMAL rather than DOUBLE, the result of the SELECT query would have contained only one row—the last one shown above.

The correct way to do floating-point number comparison is to first decide on an acceptable tolerance for differences between the numbers and then do the comparison against the tolerance value. For example, if we agree that floating-point numbers should be regarded the same if they are same within a precision of one in ten thousand (0.0001), the comparison should be written to find differences larger than the tolerance value: 

mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1
    -> GROUP BY i HAVING ABS(a - b) > 0.0001;
+------+-------+------+
| i    | a     | b    |
+------+-------+------+
|    6 | -51.4 |    0 |
+------+-------+------+
1 row in set (0.00 sec)

Conversely, to get rows where the numbers are the same, the test should find differences within the tolerance value: 

mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1
    -> GROUP BY i HAVING ABS(a - b) <= 0.0001;
+------+------+------+
| i    | a    | b    |
+------+------+------+
|    1 | 21.4 | 21.4 |
|    2 | 76.8 | 76.8 |
|    3 |  7.4 |  7.4 |
|    4 | 15.4 | 15.4 |
|    5 |  7.2 |  7.2 |
+------+------+------+
5 rows in set (0.03 sec)

Floating-point values are subject to platform or implementation dependencies. Suppose that you execute the following statements: 

CREATE TABLE t1(c1 FLOAT(53,0), c2 FLOAT(53,0));
INSERT INTO t1 VALUES('1e+52','-1e+52');
SELECT * FROM t1;

On some platforms, the SELECT statement returns inf and -inf. On others, it returns 0 and -0.

An implication of the preceding issues is that if you attempt to create a replication slave by dumping table contents with mysqldump on the master and reloading the dump file into the slave, tables containing floating-point columns might differ between the two hosts.

B.5.5 Optimizer-Related Issues

MySQL uses a cost-based optimizer to determine the best way to resolve a query. In many cases, MySQL can calculate the best possible query plan, but sometimes MySQL does not have enough information about the data at hand and has to make “educated” guesses about the data.

For the cases when MySQL does not do the "right" thing, tools that you have available to help MySQL are:

B.5.6 Table Definition-Related Issues

B.5.6.1 Problems with ALTER TABLE
B.5.6.2 TEMPORARY Table Problems

B.5.6.1 Problems with ALTER TABLE

If you get a duplicate-key error when using ALTER TABLE to change the character set or collation of a character column, the cause is either that the new column collation maps two keys to the same value or that the table is corrupted. In the latter case, you should run REPAIR TABLE on the table.

If ALTER TABLE dies with the following error, the problem may be that MySQL crashed during an earlier ALTER TABLE operation and there is an old table named A-xxx or B-xxx lying around: 

Error on rename of './database/name.frm'
to './database/B-xxx.frm' (Errcode: 17)

In this case, go to the MySQL data directory and delete all files that have names starting with A- or B-. (You may want to move them elsewhere instead of deleting them.)

ALTER TABLE works in the following way:

  • Create a new table named A-xxx with the requested structural changes.

  • Copy all rows from the original table to A-xxx.

  • Rename the original table to B-xxx.

  • Rename A-xxx to your original table name.

  • Delete B-xxx.

If something goes wrong with the renaming operation, MySQL tries to undo the changes. If something goes seriously wrong (although this shouldn't happen), MySQL may leave the old table as B-xxx. A simple rename of the table files at the system level should get your data back.

If you use ALTER TABLE on a transactional table or if you are using Windows, ALTER TABLE unlocks the table if you had done a LOCK TABLE on it. This is done because InnoDB and these operating systems cannot drop a table that is in use.

B.5.6.2 TEMPORARY Table Problems

The following list indicates limitations on the use of TEMPORARY tables:

  • A TEMPORARY table can only be of type MEMORY, MyISAM, MERGE, or InnoDB.

    Temporary tables are not supported for MySQL Cluster.

  • You cannot refer to a TEMPORARY table more than once in the same query. For example, the following does not work: 

    mysql> SELECT * FROM temp_table, temp_table AS t2;
ERROR 1137: Can't reopen table: 'temp_table'

    This error also occurs if you refer to a temporary table multiple times in a stored function under different aliases, even if the references occur in different statements within the function.

  • The SHOW TABLES statement does not list TEMPORARY tables.

  • You cannot use RENAME to rename a TEMPORARY table. However, you can use ALTER TABLE instead: 

    mysql> ALTER TABLE orig_name RENAME new_name;

  • There are known issues in using temporary tables with replication. See Section 18.4.1, “Replication Features and Issues”, for more information.

  • Temporary tables created outside stored functions and referred to across multiple calling and callee functions might result in this error: 

    ERROR 1137: Can't reopen table: 'temp_table'

B.5.7 Known Issues in MySQL

This section lists known issues in recent versions of MySQL.

For information about platform-specific issues, see the installation and porting instructions in Section 2.1, “General Installation Guidance”, and Section 26.5, “Debugging and Porting MySQL”.

The following problems are known:

  • Subquery optimization for IN is not as effective as for =.

  • Even if you use lower_case_table_names=2 (which enables MySQL to remember the case used for databases and table names), MySQL does not remember the case used for database names for the function DATABASE() or within the various logs (on case-insensitive systems).

  • Dropping a FOREIGN KEY constraint does not work in replication because the constraint may have another name on the slave.

  • REPLACE (and LOAD DATA with the REPLACE option) does not trigger ON DELETE CASCADE.

  • DISTINCT with ORDER BY does not work inside GROUP_CONCAT() if you do not use all and only those columns that are in the DISTINCT list.

  • When inserting a big integer value (between 263 and 264−1) into a decimal or string column, it is inserted as a negative value because the number is evaluated in a signed integer context.

  • With statement-based binary logging, the master writes the executed queries to the binary log. This is a very fast, compact, and efficient logging method that works perfectly in most cases. However, it is possible for the data on the master and slave to become different if a query is designed in such a way that the data modification is nondeterministic (generally not a recommended practice, even outside of replication).

    For example:

    • CREATE TABLE ... SELECT or INSERT ... SELECT statements that insert zero or NULL values into an AUTO_INCREMENT column.

    • DELETE if you are deleting rows from a table that has foreign keys with ON DELETE CASCADE properties.

    • REPLACE ... SELECT, INSERT IGNORE ... SELECT if you have duplicate key values in the inserted data.

    If and only if the preceding queries have no ORDER BY clause guaranteeing a deterministic order.

    For example, for INSERT ... SELECT with no ORDER BY, the SELECT may return rows in a different order (which results in a row having different ranks, hence getting a different number in the AUTO_INCREMENT column), depending on the choices made by the optimizers on the master and slave.

    A query is optimized differently on the master and slave only if:

    • The table is stored using a different storage engine on the master than on the slave. (It is possible to use different storage engines on the master and slave. For example, you can use InnoDB on the master, but MyISAM on the slave if the slave has less available disk space.)

    • MySQL buffer sizes (key_buffer_size, and so on) are different on the master and slave.

    • The master and slave run different MySQL versions, and the optimizer code differs between these versions.

    This problem may also affect database restoration using mysqlbinlog|mysql.

    The easiest way to avoid this problem is to add an ORDER BY clause to the aforementioned nondeterministic queries to ensure that the rows are always stored or modified in the same order. Using row-based or mixed logging format also avoids the problem.

  • Log file names are based on the server host name if you do not specify a file name with the startup option. To retain the same log file names if you change your host name to something else, you must explicitly use options such as --log-bin=old_host_name-bin. See Section 6.1.3, “Server Command Options”. Alternatively, rename the old files to reflect your host name change. If these are binary logs, you must edit the binary log index file and fix the binary log file names there as well. (The same is true for the relay logs on a slave server.)

  • mysqlbinlog does not delete temporary files left after a LOAD DATA INFILE statement. See Section 5.6.7, “mysqlbinlog — Utility for Processing Binary Log Files”.

  • RENAME does not work with TEMPORARY tables or tables used in a MERGE table.

  • When using SET CHARACTER SET, you cannot use translated characters in database, table, and column names.

  • You cannot use “_” or “%” with ESCAPE in LIKE ... ESCAPE.

  • The server uses only the first max_sort_length bytes when comparing data values. This means that values cannot reliably be used in GROUP BY, ORDER BY, or DISTINCT if they differ only after the first max_sort_length bytes. To work around this, increase the variable value. The default value of max_sort_length is 1024 and can be changed at server startup time or at runtime.

  • Numeric calculations are done with BIGINT or DOUBLE (both are normally 64 bits long). Which precision you get depends on the function. The general rule is that bit functions are performed with BIGINT precision, IF() and ELT() with BIGINT or DOUBLE precision, and the rest with DOUBLE precision. You should try to avoid using unsigned long long values if they resolve to be larger than 63 bits (9223372036854775807) for anything other than bit fields.

  • You can have up to 255 ENUM and SET columns in one table.

  • In MIN(), MAX(), and other aggregate functions, MySQL currently compares ENUM and SET columns by their string value rather than by the string's relative position in the set.

  • In an UPDATE statement, columns are updated from left to right. If you refer to an updated column, you get the updated value instead of the original value. For example, the following statement increments KEY by 2, not 1: 

    mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1;

  • You can refer to multiple temporary tables in the same query, but you cannot refer to any given temporary table more than once. For example, the following does not work: 

    mysql> SELECT * FROM temp_table, temp_table AS t2;
ERROR 1137: Can't reopen table: 'temp_table'

  • The optimizer may handle DISTINCT differently when you are using “hidden” columns in a join than when you are not. In a join, hidden columns are counted as part of the result (even if they are not shown), whereas in normal queries, hidden columns do not participate in the DISTINCT comparison.

    An example of this is: 

    SELECT DISTINCT mp3id FROM band_downloads
       WHERE userid = 9 ORDER BY id DESC;

    and 

    SELECT DISTINCT band_downloads.mp3id
       FROM band_downloads,band_mp3
       WHERE band_downloads.userid = 9
       AND band_mp3.id = band_downloads.mp3id
       ORDER BY band_downloads.id DESC;

    In the second case, using MySQL Server 3.23.x, you may get two identical rows in the result set (because the values in the hidden id column may differ).

    Note that this happens only for queries that do not have the ORDER BY columns in the result.

  • If you execute a PROCEDURE on a query that returns an empty set, in some cases the PROCEDURE does not transform the columns.

  • Creation of a table of type MERGE does not check whether the underlying tables are compatible types.

  • If you use ALTER TABLE to add a UNIQUE index to a table used in a MERGE table and then add a normal index on the MERGE table, the key order is different for the tables if there was an old, non-UNIQUE key in the table. This is because ALTER TABLE puts UNIQUE indexes before normal indexes to be able to detect duplicate keys as early as possible.

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