Slow insert in MySQL innoDB table when adding secondary key

Question

I have a buffing problem with an InnoDB table that I cannot figure out:

• I have a table with 5 columns, of which 4 columns make up the primary key and there are 3 more secondary keys; in that table bulk inserts take very long time - up to 400 sec for 100,000 records (that is just 250 records/sec)
• If all secondary indexes are dropped, the inserts are very fast - 20 sec for 100,000 records (5000 records/sec or 20 times faster)
• If I add back even one secondary key, on a single bigint user-id column which is also a part of the primary key (I often need to search for the data of specific users), the inserts immediately become 3-4 times slower.

What is going on here? I suspect the answer is in the way InnoDB stores indexes, but after a lot of reading, I can't figure it out. What's weirdest of all is that the same table, with all the indexes added, with MyISAM engine always works at 5000 records/sec.

Answer 1

I don't know why. But the MySQL website provides a performance tuning tip for inserting into InnoDB with secondary keys (which implies to me that this is just the way it is and you have to deal with it): "If you have UNIQUE constraints on secondary keys, you can speed up table imports by temporarily turning off the uniqueness checks during the import session..."