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15 Reasons Why a Query That Was Fast Yesterday Is Slow Today

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Пересказ статьи Brent Ozar. 15 Reasons Your Query Was Fast Yesterday, But Slow Today

In order based on the frequency with which I observe them:

1. Different workload on the server (for example, a backup process is running).
2. You get a different query plan due to parameter sniffing.
3. The query has changed, for example, someone updated the code on the server or added a column to the select clause.
4. You got a different plan because of a stat change.
5. The hardware has changed (perhaps someone changed the hardware settings, moved to a nearby noisy virtual machine, or there is hardware competition in the cloud).
6. Someone added or removed an index.
7. Someone added or removed a trace flag or other server-level configuration parameter.
8. The application itself has changed, and is not able to process the query results so quickly.
9. We put in a patch and it had an unexpected side effect.
10. You have the same plan, but different memory allocation.
11. Someone is modifying a large number of rows in a single transaction, and you are caught in a lock escalation.
12. Your request strangled the Resource Governor.
13. He was initially not fast.

And since SQL Server 2017 (if you had columnar indexes) and SQL Server 2019 (for all):
14. You get different memory allocations due to adaptive memory grants.
15. You get a different connection type due to the change in the adaptive connection threshold.

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