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
--tmpdir option to
mysqld to specify a directory in a file
system where you have enough space.
In MySQL 5.5, 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.
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 --tmpdir to
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
LOAD DATA
INFILE operations. If files in the temporary file
directory are lost when the server restarts, replication
fails.
MySQL creates all temporary files as hidden files. This ensures that the temporary files are removed if mysqld is terminated. The disadvantage of using hidden files is that you do not see a big temporary file that fills up the file system in which the temporary file directory is located.
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_*.
ALTER TABLE creates a temporary
table in the same directory as the original table.