Keep Predicates Meaningful

Gail Shaw shows that adding a valueless predicate to change a scan operation into a seek operation does not guarantee a performance improvement:

I remember a forum thread from a while back. The question was on how to get rid of the index scan that was in the query plan. Now that’s a poor question in the first place, as the scan might not be a problem, but it’s the first answer that really caught my attention.

Since the primary key is on an identity column, you can add a clause like ID > 0 to the query, then SQL will use an index seek.

Technically that’s correct. If the table has an identity column with the default properties (We’ll call it ID) and the clustered index is on that identity column, then a WHERE clause of the form WHERE ID > 0 AND <any other predicates on that table> can indeed execute with a clustered index seek (although it’s in no way guaranteed to do so). But is it a useful thing to do?

Time for a made up table and a test query.

Anything Gail writes is a must-read; this is no exception.

Related Posts

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

When Table Variables Have Realistic Estimates, Unrealistic Results May Occur

Milos Radivojevic wraps up a series on deferred compilation for table variables by looking at a hack which used to work but no longer does: With this change, the query is executed very fast, with the appropriate execution plan: SQL Server Execution Times: CPU time = 31 ms,  elapsed time = 197 ms. However, the LOOP hint […]

Read More

Categories

February 2016
MTWTFSS
« Jan Mar »
1234567
891011121314
15161718192021
22232425262728
29