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Re: query scanning all rows
Posted by: Rick James
Date: September 15, 2012 12:44PM

select  te.topic_id,te.entry_id
    from  topic_entry te
    WHERE  te.topic_id in (
        SELECT  topic_id
            from  topic_user
            where  user_id ='xyz')
      AND  te.level=4
NO! Don't use "IN ( SELECT ... )"; it does not optimize well (until 5.6.5).

 
select  te.topic_id, te.entry_id
    from  topic_entry te
    JOIN  topic_user tu  ON tu.topic_id = te.topic_id
    where  tu.user_id ='xyz'
      AND  te.level=4
Also helpful:
On topic_users:
KEY `user_id` (`user_id`), -->
KEY `user_id` (`user_id`, topic_id),

How many levels are there? How many rows have level=4? If the cardinality is poor, then this won't be used:
KEY `level` (`level`),

After making those changes, I expect "EXPLAIN SELECT ..." to show that it is picking topic_user as the first table, and say only a few "Rows" are needed.

Lesser issues:
Do you really need BIGINT? INT UNSIGNED lets you get up to 4 billion, and saves 4 bytes per row. (See also MEDIUMINT, etc)

> `entry_object` text,
If you can live with VARCHAR(255), it will perform slightly better.

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