Any way to avoid a filesort when order by is different to where clause?

Question

I have an incredibly simple query (table type InnoDb) and EXPLAIN says that MySQL must do an extra pass to find out how to retrieve the rows in sorted order.

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Answer 1

Note that the MySQL term filesort doesn't necessarily mean it writes to the disk. It just means it's going to sort without using an index. If the result set is small enough, MySQL will sort it in memory, which is orders of magnitude faster than disk I/O.

You can increase the amount of memory MySQL allocates for in-memory filesorts using the sort_buffer_size server variable. In MySQL 5.1, the default sort buffer size is 2MB, and the maximum you can allocate is 4GB.

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update: Regarding Jonathan Leffler's comment about measuring how long the sorting takes, you can learn how to use SHOW PROFILE FOR QUERY which will give you the breakdown of how long each phase of query execution takes.

Answer 2

Try adding a combined index on (commentable_id, created_at).