问题
in High Performance MySQL on page 159 they talk about breaking up complex queries into simple ones:
Converting
SELECT * FROM tag
JOIN tag_post ON tag_post.tag_id=tag.id
JOIN post ON tag_post.post_id=post.id
WHERE tag.tag='mysql';
To
SELECT * FROM tag WHERE tag='mysql';
SELECT * FROM tag_post WHERE tag_id=1234;
SELECT * FROM post WHERE post.id in (123,456,567,9098,8904);
And sort of doing the actual join yourself in your application.
My Question is wether this is stil such a good idea when the final query has a where-clause with a few thousand IDs it needs to match (the actual table itself has about 500k entries).
What I mean is, will there be a big penalty for having a query like
SELECT * FROM post WHERE post.id in (123,456,567, ... <a few thousand IDs here> ... ,9098,8904);
instead of the join-statement above? Would it help to move this logic to Stored Procedures inside the Database (while considering how poorly stored procedures are implemented in MySQL)?
回答1:
Join decomposition is useful in certain situations, but in most situations the joins are going to be faster.
In your case, I would stick with the joins instead of passing in a few thousand IDs in an IN clause.
来源：https://stackoverflow.com/questions/5385059/myisam-performance-join-decomposition