Understanding Query Durations

Kendra Little explains some of the intricacies behind query durations:

I typically look at the ‘CPU time’ metric when tuning instead of ‘elapsed time’ (duration). This can work well for tuning because you’re measuring how much more efficient you made the  query in terms of CPU cycles.

But ‘CPU time’ isn’t perfect, and it can get a little weird for reporting results to users, because:

  • If the query uses parallelism, CPU time can be higher than the duration — which may make the query seem “slower” than it actually is to anyone reading a report

  • ‘elapsed time’ includes all the time that it takes to display the results in Management Studio, which is probably a different duration than it would take to return the results to an application server. If you’re just returning a few rows, this may be negligible– but once it gets into the thousands of rows, it can be very noticeable.

Moral of the story:  also use SQL Sentry Plan Explorer…

Related Posts

Actual Versus Estimated Rows In SSMS 18

David Alcock is happy with a feature in SQL Server Management Studio 18: Last week Microsoft released SQL Server Management Studio 18.0 into public preview, here’s a link so you can read about the new and improved functionality it offers. One significant change is the addition of actual vs estimated row counts onto the showplan operators in execution […]

Read More

Risks Of Using Resource Governor To Set Max DOP

Joe Obbish builds an example where Resource Governor’s CPU cap can actively harm query performance: I uploaded the query plan here if you want to look at it. This type of scenario can happen even without Resource Governor. For example, a compiled parallel query may be downgraded all the way to MAXDOP 1 if it can’t get […]

Read More

Categories

September 2016
MTWTFSS
« Aug Oct »
 1234
567891011
12131415161718
19202122232425
2627282930