Syntax:
OPTIMIZE [NO_WRITE_TO_BINLOG | LOCAL] TABLE
    tbl_name [, tbl_name] ...

Reorganizes the physical storage of table data and associated index
data, to reduce storage space and improve I/O efficiency when accessing
the table. The exact changes made to each table depend on the storage
engine used by that table. This statement does not work with views.

Use OPTIMIZE TABLE in these cases, depending on the type of table:

o After doing substantial insert, update, or delete operations on an
  InnoDB table that has its own .ibd file because it was created with
  the innodb_file_per_table option enabled. The table and indexes are
  reorganized, and disk space can be reclaimed for use by the operating
  system.

o After deleting a large part of a MyISAM or ARCHIVE table, or making
  many changes to a MyISAM or ARCHIVE table with variable-length rows
  (tables that have VARCHAR, VARBINARY, BLOB, or TEXT columns). Deleted
  rows are maintained in a linked list and subsequent INSERT operations
  reuse old row positions. You can use OPTIMIZE TABLE to reclaim the
  unused space and to defragment the data file. After extensive changes
  to a table, this statement may also improve performance of statements
  that use the table, sometimes significantly.

This statement requires SELECT and INSERT privileges for the table.

OPTIMIZE TABLE is also supported for partitioned tables. For
information about using this statement with partitioned tables and
table partitions, see
http://dev.mysql.com/doc/refman/5.5/en/partitioning-maintenance.html.

URL: http://dev.mysql.com/doc/refman/5.5/en/optimize-table.html

