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MySQL uses the value of the TMPDIR
environment variable as
the pathname of the directory in which to store temporary files. If you don't
have TMPDIR
set, MySQL uses the system default, which is
normally `/tmp' or `/usr/tmp'. If the filesystem containing your
temporary file directory is too small, you should edit mysqld_safe
to
set TMPDIR
to point to a directory in a filesystem where you have
enough space! You can also set the temporary directory using the
--tmpdir
option to mysqld
.
MySQL creates all temporary files as hidden files. This ensures
that the temporary files will be removed if mysqld
is terminated. The
disadvantage of using hidden files is that you will not see a big temporary
file that fills up the filesystem 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 needed is:
(length of what is sorted + sizeof(database pointer)) * number of matched rows * 2 |
sizeof(database pointer)
is usually 4, 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.
If you use MySQL 4.1 or later you can spread load between
several physical disks by setting --tmpdir
to a list of paths
separated by colon :
(semicolon ;
on Windows). They
will be used in round-robin fashion.
Note: These paths should end up on different physical disks,
not different partitions of the same disk.
It is possible to set tmpdir
to point to a memory-based filesystem,
except if the MySQL server is a slave. If it is a slave, it needs some
of its temporary files (for replication of temporary tables or of
LOAD DATA INFILE
) to survive a machine's reboot, so a
memory-based tmpdir
which is cleared when the machine reboots is not
suitable; a disk-based tmpdir
is necessary.
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