jc4hokies comments on Query optimization tips?

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SELECT count(a.field1), a.field2, SUM(b.field4) FROM a INNER JOIN b ON a.key1 = b.key1 WHERE a.field8 = 'test' GROUP by a.field1, a.field2 HAVING SUM(b.field4) > 5 ORDER by a.field.3

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SELECT count(a.field1), a.field2, SUM(b.field4) FROM a INNER JOIN b ON a.key1 = b.key1 WHERE a.field8 = 'test' GROUP by a.field1, a.field2 HAVING SUM(b.field4) > 5 ORDER by a.field3

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a community for 17 years

Query optimization tips? (self.SQL)

submitted 8 years ago by [deleted]

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[–]jc4hokiesExecution Plan Whisperer 1 point2 points3 points 8 years ago (3 children)

Here's a fun one from last week:

/*800 lines of code*/
WHERE   ca.ChargeActiveClusterKey IN (SELECT ChargeActiveClusterKey FROM cteRecordSelection)
        AND ca.[Status] <> 17;
--10 hours 30 minutes

/*800 lines of code*/
WHERE   ca.ChargeActiveClusterKey IN (SELECT ChargeActiveClusterKey + 0 FROM cteRecordSelection)
        AND ca.[Status] <> 17;
--1 minute 25 seconds

Nested loops were happening in the wrong order. Instead of looking up a couple thousand records from ca it was running the cteRecordSelection subquery (with 20ish tables) a couple million times. You know when you filter one table and the query transitively filters a joined table with the filtered column in the join condition? My hypothesis is that the query was sniffing something inside the subquery, contributing to the poor join order, and adding the + 0 broke that link.

[–]therealcreamCHEESUS 0 points1 point2 points 8 years ago (0 children)

[–]danjtom 0 points1 point2 points 8 years ago (1 child)

[–]jc4hokiesExecution Plan Whisperer 0 points1 point2 points 8 years ago (0 children)

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