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, Netware and OS2, MySQL checks in order the values
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
Starting from MySQL 4.1, 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
;”) on Windows,
NetWare, and OS/2.
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 not set
point to a directory 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
INFILE operations. If files in the temporary file
directory are lost when the server restarts, replication
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.
SELECT queries, MySQL
also creates temporary SQL tables. These are not hidden and
have names of the form
ALTER TABLE creates a temporary
table in the same directory as the original table.