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When comparing the execution time of two different queries, it's important to clear the cache to make sure that the execution of the first query does not alter the performance of the second.

In a Google Search, I could find these commands:

DBCC FREESYSTEMCACHE
DBCC FREESESSIONCACHE
DBCC FREEPROCCACHE

In fact, my queries are taking a more realistic time to complete after several executions than before. However, I'm not sure this is the recommended technique.

What's the best practice?

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migrated from stackoverflow.com Jan 17 '12 at 17:49

This question came from our site for professional and enthusiast programmers.

up vote 24 down vote accepted

Personally, for a common query the 2nd and subsequent executions matter more.

Are you testing disk IO or query performance?

Assuming your query runs often and is critical, then you want to measure that under real life conditions. And you don't want to clear prod server caches each time...

If you want, you can:

  • DBCC DROPCLEANBUFFERS clears clean (unmodified) pages from the buffer pool
    Precede that with a CHECKPOINT to flush any dirty pages to disk first
  • DBCC FLUSHPROCINDB clears execution plans for that database

Also see (on DBA.SE)

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I was always told to use:

dbcc dropcleanbuffers;

From MSDN:

Use DBCC DROPCLEANBUFFERS to test queries with a cold buffer cache without shutting down and restarting the server.

To drop clean buffers from the buffer pool, first use CHECKPOINT to produce a cold buffer cache. This forces all dirty pages for the current database to be written to disk and cleans the buffers. After you do this, you can issue DBCC DROPCLEANBUFFERS command to remove all buffers from the buffer pool.

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Plus: DBCC FREEPROCCACHE to clear any cached execution plans... – marc_s Nov 28 '11 at 12:40
1  
Only if you want to test IO, surely... – gbn Nov 28 '11 at 12:44

Late answer but may be of use to other readers

DBCC DROPCLEANBUFFERS is an often used command for query testing and gauging query execution speed. This command (when run) leaves behind only the dirty pages, which is actually a small portion of data. It removes all the clean pages for an entire server.

Be advised that this command should not be run on production environment. Running this command will result in mostly empty buffer cache. Running any query after executing the DBCC DROPCLEANBUFFERS command, will use physical reads to bring back the data into the cache, which is very likely going to be a lot slower than memory.

Again, treat this command similarly to DBCC FREEPROCCACHE - it should not be run on any production server unless you absolutely know what you are doing.

This can be a useful development tool because you can run a query in a performance testing environment over and over without any changes in speed/efficiency due to caching of data in memory.

Learn more at: http://www.sqlshack.com/insight-into-the-sql-server-buffer-cache/

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