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at November 2, 2009 07:13 by kurokikaze
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SELECT t.TABLE_SCHEMA AS `db`, t.TABLE_NAME AS `table`, s.INDEX_NAME AS `index name`, s.COLUMN_NAME AS `field name`, s.SEQ_IN_INDEX `seq in index`, s2.max_columns AS `# cols`, s.CARDINALITY AS `card`, t.TABLE_ROWS AS `est rows`, ROUND(((s.CARDINALITY / IFNULL(t.TABLE_ROWS, 0.01)) * 100), 2) AS `sel %` FROM INFORMATION_SCHEMA.STATISTICS s INNER JOIN INFORMATION_SCHEMA.TABLES t ON s.TABLE_SCHEMA = t.TABLE_SCHEMA AND s.TABLE_NAME = t.TABLE_NAME INNER JOIN ( SELECT TABLE_SCHEMA, TABLE_NAME, INDEX_NAME, MAX(SEQ_IN_INDEX) AS max_columns FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA != 'mysql' GROUP BY TABLE_SCHEMA, TABLE_NAME, INDEX_NAME ) AS s2 ON s.TABLE_SCHEMA = s2.TABLE_SCHEMA AND s.TABLE_NAME = s2.TABLE_NAME AND s.INDEX_NAME = s2.INDEX_NAME WHERE t.TABLE_SCHEMA != 'mysql' /* Filter out the mysql system DB */ AND t.TABLE_ROWS > 10 /* Only tables with some rows */ AND s.CARDINALITY IS NOT NULL /* Need at least one non-NULL value in the field */ AND (s.CARDINALITY / IFNULL(t.TABLE_ROWS, 0.01)) < 1.00 /* unique indexes are perfect anyway */ ORDER BY `sel %`, s.TABLE_SCHEMA, s.TABLE_NAME /* DESC for best non-unique indexes */ LIMIT 10;
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http://www.slideshare.net/ZendCon/sql-query-tuning-the-legend-of-drunken-query-master-presentation
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Select indexes with lowest selectivity. By Jay Pipes.
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Get low selectivity indexes from DB schema
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MySQL