(mysql.info) optimize-table

(mysql.info) table-maintenance-sql
 
 13.5.2.5 `OPTIMIZE TABLE' Syntax
 ................................
 
      OPTIMIZE [LOCAL | NO_WRITE_TO_BINLOG] TABLE TBL_NAME [, TBL_NAME] ...
 
 `OPTIMIZE TABLE' should be used if you have deleted a large part of a
 table or if you have made many changes to a 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.
 
 In most setups, you need not run `OPTIMIZE TABLE' at all. Even if you
 do a lot of updates to variable-length rows, it is not likely that you
 need to do this more than once a week or month and only on certain
 tables.
 
 `OPTIMIZE TABLE' works only for `MyISAM', `BDB', and `InnoDB' tables.
 
 For `MyISAM' tables, `OPTIMIZE TABLE' works as follows:
 
   1. If the table has deleted or split rows, repair the table.
 
   2. If the index pages are not sorted, sort them.
 
   3. If the table's statistics are not up to date (and the repair could
      not be accomplished by sorting the index), update them.
 
 For `BDB' tables, `OPTIMIZE TABLE' currently is mapped to `ANALYZE
 TABLE'. See  analyze-table.
 
 For `InnoDB' tables, `OPTIMIZE TABLE' is mapped to `ALTER TABLE', which
 rebuilds the table to update index statistics and free unused space in
 the clustered index.
 
 You can make `OPTIMIZE TABLE' work on other storage engines by starting
 `mysqld' with the -skip-new or -safe-mode option. In this case,
 `OPTIMIZE TABLE' is just mapped to `ALTER TABLE'.
 
 `OPTIMIZE TABLE' returns a result set with the following columns:
 
 *Column*    *Value*
 `Table'     The table name
 `Op'        Always `optimize'
 `Msg_type'  One of `status', `error', `info', or
             `warning'
 `Msg_text'  The message
 
 Note that MySQL locks the table during the time `OPTIMIZE TABLE' is
 running.
 
 `OPTIMIZE TABLE' statements are written to the binary log unless the
 optional `NO_WRITE_TO_BINLOG' keyword(or its alias `LOCAL') is used.
 This is done so that `OPTIMIZE TABLE' statements used on a MySQL server
 acting as a replication master will be replicated by default to the
 replication slave.
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