( Temporary files

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 A.4.4 Where MySQL Stores Temporary Files
 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', `/var/tmp', or `/usr/tmp'.  If the filesystem
 containing your temporary file directory is too small, you can use the
 `--tmpdir' option to `mysqld' to specify a directory in a filesystem
 where you have enough space.
 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 characters
 (`;') 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
 `--tmpdir' to point to a directory on a memory-based filesystem 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 will fail.
 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 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.
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