When UNION ALL Can Beat OR

Bert Wagner compares a couple methods for writing a query:

Suddenly those key-lookups become too expensive for SQL Server and the query optimizer thinks it’ll be faster to just scan the entire clustered index.

In general this makes sense; SQL Server tries to pick plans that are good enough in most scenarios, and in general I think it chooses wisely.

However, sometimes SQL Server doesn’t pick great plans. Sometimes the plans it picks are downright terrible.

If that particular topic is interesting, I’ve a blog post from a few years back on a similar vein.

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 2018
MTWTFSS
« Jan Mar »
 1234
567891011
12131415161718
19202122232425
262728