Performance Tuning - Do you make sure to clear SQL server cache when performing benchmark tests?

Last updated by Tiago Araújo [SSW] about 2 years ago.See history

When you are tuning SQL statements you tend to play in SQL management studio for a while. During this time SQL caches your query's and execution plans.

All well and good but when you are trying to speed up an existing query that is taking some time then you may not be making a difference even though your execution times are way down.

You really need to clear SQL's cache (or buffer) every time you test the speed of a query. This prevents the data and/or execution plans from being cached, thus corrupting the next test.

To clear SQL Server's cache, run DBCC DROPCLEANBUFFERS, which clears all data from the cache. Then run DBCC FREEPROCCACHE, which clears the stored procedure cache.

ClearSQLServerCache BenchmarkTests
Figure: First call is after clearing the cache, the second one is without clearing the cache (26 seconds vs 2 seconds)

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