Question

I am using MySQL, I have a dump file, the content of it are something like following

TRUNCATE TABLE cars;

ALTER TABLE cars DISABLE KEYS;

INSERT INTO cars ...    ;

ALTER TABLE cars ENABLE KEYS;

OPTIMIZE TABLE cars

I am wondering what are the following statements(from the above dump) doing respectively:

1.

ALTER TABLE cars DISABLE KEYS;

2.

ALTER TABLE cars ENABLE KEYS;

3.

OPTIMIZE TABLE cars

Answer 1

DISABLE KEYS tells mySQL not to update indexes while you are inserting. ENABLE KEYS tells it to recreate and then begin updating indexes again. Lastly OPTIMIZE does a number of things including updating statistics and sorting index pages. This is important after large inserts/updates/deletes.

The code above would typically be used for a bulk insert/update. Disabling indexes helps the performance of large insert/update/delete operations.