Steve Fenton

Testing SQL query performance

I was asked about how to test a query without all of the clever caching that SQL performs, this is how you do it… but read on for important information.

DBCC DROPCLEANBUFFERS
DBCC FREEPROCCACHE 
GO

SELECT * FROM MyTable

Very important notes

There are some very important things to consider before you use these commands.

• You almost certainly don’t want to run this on a live server as you’ll force everything to be read from disk and things will be slow
• You almost certainly don’t want to test your queries in this way if you are opimizing them… see below

How To really test queries

Queries run in “cached mode” more than they run on an empty cache, so you ought to be testing them cached as this is “real life”. Emptying the cache every time is a useful test of disk access, but not a useful test of a SQL query. So here is how I test my queries…

SET STATISTICS TIME ON
GO

SELECT * FROM MyTable
GO

SET STATISTICS TIME OFF
GO

Basically, it is just the plain query, with statistics supplied in the “Messages” window.