OPTIMZE 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, BLOB or TEXT columns). Deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. You can use OPTIMIZE TABLE to reclaim the unused space.

OPTIMIZE TABLE works by making a temporary copy of the original table. The old table is copied to the new table (without the unused rows), then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. While OPTIMIZE TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready.