By: Kuroki Kaze

Kuroki Kaze — Wed, 02 Dec 2009 17:23:57 +0000

You can also find tables with non-efficient (low cardinality) indexes by running:

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;

Comments on: Finding and Fixing Slow MySQL Queries

By: Kuroki Kaze