Am 19.03.2014 16:27, schrieb Morgan Tocker: > Reindl, you can call me Harald
>>> The order of the WHERE clause does not matter >> >> that is simply not true >> >> otherwise that documentation would not exist >> http://dev.mysql.com/doc/refman/5.5/en/multiple-column-indexes.html > > Yes, the order of the columns in composite indexes *does matter*. This is a > different question to what Christophe asked. the question was "is there any performance impact" and (DWProcessed / Lowest cardinality and indexed) so *yes* it does matter, i have seen MySQL more than once not using any index because a unlucky order of where clauses up to copy a 16 GB table in a table file instead using a existing primary key _______________________________ SELECT * FROM Status WHERE DWProcessed = 0 AND PreviousStatus NOT IN ('PENDING', 'ACCEPTED') AND SubscribeDate < DATE_SUB(NOW(), INTERVAL 24 HOUR); I'd like the first filter (DWProcessed / Lowest cardinality and indexed) Is there any performance impact on query processing, about the order of WHERE clauses ?
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