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(mysql.info)Temporary files

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Where MySQL Stores Temporary Files
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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 file system containing your
temporary file directory is too small, you should edit `safe_mysqld' to
set `TMPDIR' to point to a directory in a file system 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 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 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.