Re: Slow query. Why is it slow?

[ben jaz]

Rick James Wrote:
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> sql.html is nice. What did you use to format it?



Hi Rick;

Thank you for your post. I use http://www.dpriver.com/pp/sqlformat.htm to format sql. And for explain boxes I just copy paste the code for the table :)

Rick I have go t another question for you:

Query # 7 takes 0.0020 seconds; Its here: http://webmoosh.com/sql.html#q7
Query# 9 on the other hand takes 5.5042 sec http://webmoosh.com/sql.html#q9

Why in query 7 Mysql chooses PRIMARY and in query 9 it uses category_index (and the query 9 is slow)? If you look at indexes you see that category_index and class_index both have exact cardinality of 17!

What should I do so I can use exact same query for both cases? Drop category_index? Now I found out that I can FORCE INDEX (PRIMARY) and that way the query takes 0.0003 seconds but that's not right!

Indexes: http://webmoosh.com/indexes.png

Thanks Rick