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(mysql.info)Symbolic links to tables

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Using Symbolic Links for Tables
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Before MySQL 4.0 you should not symlink tables, if you are not very
carefully with them. The problem is that if you run `ALTER TABLE',
`REPAIR TABLE' or `OPTIMIZE TABLE' on a symlinked table, the symlinks
will be removed and replaced by the original files. This happens
because the above command works by creating a temporary file in the
database directory and when the command is complete, replace the
original file with the temporary file.

You should not symlink tables on system that doesn't have a fully
working `realpath()' call. (At least Linux and Solaris support
`realpath()')

In MySQL 4.0 symlinks is only fully supported for `MyISAM' tables.  For
other table types you will probably get strange problems when doing any
of the above mentioned commands.

The handling of symbolic links in MySQL 4.0 works the following way
(this is mostly relevant only for `MyISAM' tables).

   * In the data directory you will always have the table definition
     file and the data/index files.

   * You can symlink the index file and the data file to different
     directories independent of the other.

   * The symlinking can be done from the operating system (if `mysqld'
     is not running) or with the `INDEX/DATA DIRECTORY="path-to-dir"'
     command in `CREATE TABLE'. Note: CREATE TABLE.

   * `myisamchk' will not replace a symlink with the index/file but
     work directly on the files the symlinks points to. Any temporary
     files will be created in the same directory where the data/index
     file is.

   * When you drop a table that is using symlinks, both the symlink and
     the file the symlink points to is dropped.  This is a good reason
     to why you should NOT run `mysqld' as root and not allow persons
     to have write access to the MySQL database directories.

   * If you rename a table with `ALTER TABLE RENAME' and you don't
     change database, the symlink in the database directory will be
     renamed to the new name and the data/index file will be renamed
     accordingly.

   * If you use `ALTER TABLE RENAME' to move a table to another
     database, then the table will be moved to the other database
     directory and the old symlinks and the files they pointed to will
     be deleted.

   * If you are not using symlinks you should use the `--skip-symlink'
     option to `mysqld' to ensure that no one can drop or rename a file
     outside of the `mysqld' data directory.

Things that are not yet supported:

   * `ALTER TABLE' ignores all `INDEX/DATA DIRECTORY="path"' options.

   * `CREATE TABLE' doesn't report if the table has symbolic links.

   * `mysqldump' doesn't include the symbolic links information in the
     output.

   * `BACKUP TABLE' and `RESTORE TABLE' don't respect symbolic links.