Performance of MYSQL "IN"

Question

Hi all,

I'm running a MYSQL query in two steps. First, I get a list of ids with one query, and then I retrieve the data for those ids using a second query along the lines of SELECT * FROM data WHERE id in (id1, id2 ...). I know it sounds hacky, but I've done it this way as the queries are very complicated; the first involves lots of geometry and triggernometry, the second one lots of different joins. I'm sure they could be written in a single query, but my MYSQL isn't good enough to pull it off.

This approach works, but it doesn't feel right; plus I'm concerned it won't scale. At the moment I am testing on a database of 10,000 records, with 400 ids in the "IN" clause ( i.e. IN (id1, id2 ... id400) ) and performance is fine. But what if there are say 1,000,000 records?

Where are the performance bottlenecks (speed, memory, etc) for this kind of query? Any ideas for how to refactor this kind of query for be awesome too. (for example, if it is worth swotting up on stored procedures).

Answer 1

Starting from a certain number of records, the IN predicate over a SELECT becomes faster than that over a list of constants.

See this article in my blog for performance comparison:

• Passing parameters in MySQL: IN list vs. temporary table

If the column used in the query in the IN clause is indexed, like this:

SELECT  *
FROM    table1
WHERE   unindexed_column IN
        (
        SELECT  indexed_column
        FROM    table2
        )

, then this query is just optimized to an EXISTS (which uses but a one entry for each record from table1)

Unfortunately, MySQL is not capable of doing HASH SEMI JOIN or MERGE SEMI JOIN which are yet more efficient (especially if both columns are indexed).

Answer 2

Why do you extract the ids first? You should probably just join the tables. If you use the ids for something else, you can insert them in a temp table before and use this table for the join.