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Re: Distinct/group by, order by and filesort
Posted by: Rick James
Date: February 25, 2010 10:39AM

The parens in this are confusing:
SELECT DISTINCT( article.id ) as TD, article.*
That is probably identical to
SELECT DISTINCT article.id as TD, article.*
That is, the DISTINCT is applied to the whole row, not just article.id.

Usually I admonish people to change subqueries into JOINs. This might be a case where going the other way is better.

SELECT a2.id as TD, a2.*, p.*
    FROM article a2,
         press_release p,
         ( SELECT id FROM article
                 WHERE published = 1
                   AND TORM_IS_CURRENT_REVISION = 1
         ) a1
    WHERE a2.id = a1.id
      AND a2.id = p.id
    ORDER BY a2.article_date;
Notes:
* The subquery will be "Using index" because of acx10.
* If id is UNIQUE, then no DISTINCT is needed. but it is not declared UNIQUE; why?
* There are a lot of redundant indexes. When one non-UNIQUE INDEX is a prefix of another, DROP the shorter one.
* TORM_IS_CURRENT_REVISION seems to be NULLable -- perhaps a mistake?
* I hope press_release has an index starting with (id).

To assist in analyzing slow SELECTs, please provide
* SHOW CREATE TABLE tbl\G -- engine, indexes (easier for me to read)
* SHOW TABLE STATUS LIKE 'tbl'\G -- sizes
* SHOW VARIABLES LIKE '%buffer%'; -- cache size
and surround them with [ code ] and [ / code ]

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Re: Distinct/group by, order by and filesort
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