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Re: Queries slow down daily, optimizing helps -> why?
Posted by: Rick James
Date: February 27, 2009 07:56PM

More details may be in the slowlog.

Your earlier attempts probably said 24K "Rows examined". This was a full scan of one table, plus probing the other table for every row in the first one.

With the new index, "Rows examined" is probably about 13K -- full scan of one table, filtering out most of the rows, then probing the other table only for the remaining rows.

I would not make a general rule on which table should be "on top" in Explain. There are a zillion things that Explain might say; I enjoy explaining one EXPLAIN at a time. You may find a dozen such postings in this forum, plus Newbie and Optimizer forums.

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