On Unix, MySQL uses the value of the
environment variable as the path name of the directory in
which to store temporary files. If
is not set, MySQL uses the system default, which is usually
On Windows, MySQL checks in order the values of the
TMP environment variables. For the first
one found to be set, MySQL uses it and does not check those
remaining. If none of
TMP are set,
MySQL uses the Windows system default, which is usually
--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.
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 can
option to specify a separate directory for holding temporary
files when replicating
DATA statements. This directory should be in a
disk-based file system (not a memory-based file system) so
that the temporary files used to replicate LOAD DATA can
survive machine restarts. The directory also should not be one
that is cleared by the operating system during the system
startup process. However, replication can now continue after a
restart if the temporary files have been removed.
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
BY), MySQL normally uses one or two temporary files.
The maximum disk space required is determined by the following
(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 statements, MySQL creates temporary SQL tables that
are not hidden and have names that begin with
SELECT queries creates
temporary SQL tables to hold intermediate results.
DDL operations that rebuild the table and are not performed
online using the
technique create a temporary copy of the original table in the
same directory as the original table.
Online DDL operations may use temporary log files for recording concurrent DML, temporary sort files when creating an index, and temporary intermediate tables files when rebuilding the table. For more information, see Section 14.13.3, “Online DDL Space Requirements”.
See also Orphan Temporary Tables.