Performance Impact With Nullable Columns

Daniel Hutmacher shows us situations in which nullable columns can lead to worse performance:

We expected the Semi Join to turn into an Anti Semi Join, but the plan now also contains a Nested Loop branch with a Row Count Spool – what’s that about? Turns out the Row Count Spool, along with its index seek, has to do with the NOT IN() and the fact that we’re looking at a nullable column. Remember that…
x NOT IN (y, z, NULL)
… always returns false, because the NULL value could represent essentially anything, including the x. And so it is with the inner table, if there happens to be a NULL value among those rows.

Definitely worth the read. There are ways around this performance hit but design decisions have consequences.

Related Posts

Replaying Workloads with WorkloadTools

Gianluca Sartori shows an example of using the WorkloadTools application to replay a workload, including where the analytics server cannot directly access the production database: Regardless of the method that you decided to use, at the end of the replays, you will have two distinct sets of tables containing the workload analysis data, sitting in […]

Read More

Troubleshooting Query Performance Changes

Erin Stellato walks us through a troubleshooting guide when users complain about poorly-performing queries: This is tale of troubleshooting… When you unexpectedly or intermittently encounter a change in query performance, it can be extremely frustrating and troublesome for a DBA or developer. If you’re not using Query Store, a third-party application, or your own method […]

Read More

Categories

January 2019
MTWTFSS
« Dec Feb »
 123456
78910111213
14151617181920
21222324252627
28293031