Fast Queries Can Hurt Performance

Andy Mallon diagnoses a fast query which was hurting performance:

This code is coming from an ORM, which is parameterizing the filters, but not the (unnecessary & arbitrary) TOP value. The DMVs all viewed these are separate queries, so it was not aggregating the stats. It also wasn’t reusing the plan (thus chewing up even more CPU from frequent compiles). If the TOP had not been there, or it had been passed as a parameter, my initial query of sys.dm_exec_query_stats should have found it.

There are a couple of issues Andy works through, and his advice is good:  just because something runs quickly doesn’t mean it can’t (in aggregate) have a negative effect on your server.

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Optimizer Imperfections With Complex Filters

Erik Darling shows a couple examples of how the optimizer will sometimes pick a superior plan when dealing with complicated filters but not always: Sometimes, the optimizer can take a query with a complex where clause, and turn it into two queries. This only happens up to a certain point in complexity, and only if […]

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Hiding Work: The Nested Loop Operator

Erik Darling explains that the nested loop operator is like a duck: there’s more going on beneath the surface than it lets on: I’m going to talk about my favorite example, because it can cause a lot of confusion, and can hide a lot of the work it’s doing behind what appears to be a […]

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