MyISAM Performance: Join Decomposition?

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开发者_开发百科 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)?

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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.