Be careful with table variables

I just looked into a poorly performing stored procedure and brought it down from over 4 minutes to under 4 seconds by making one simple change: converting a table variable into a temporary table. What’s interesting is that there were only about 20,000 rows of about 1,100 bytes each in the table variable, which would be nothing in a permanent table. So what happened? I got stuck in and this is what I found.
The stored proc is for a sales report where the user can choose from a list of things to group the results by. The relevant sales are selected into a table variable with an empty column Group1, the approriate column is copied into Group1, and then there’s a SELECT with a whole lot of aggregations GROUPed BY Group1. The query plan showed 99% of the cost was in the main insert into the table variable. Further reading revealed that no statistics are created on a table variable and so to the Query Optimizer it always has a cardinality of 1, whereas statistics are created on temporary tables. Was this the problem?
I changed it to use a temporary table instead, ran that, and compared the query plans and the output from the Profiler, including the events Auto Stats (under Performance) and SP:StmtStarting & SP:StmtCompleted (under Stored Procedures). The query plans for populating the temporary table and table variable were identical but the temporary table approach showed statistics being created on the temporary table before the final SELECT, and the SELECT itself was parallelized. The SP:StmtStarting and SP:StmtCompleted events in the Profiler confirmed that it was the final SELECT that took nearly all the time with a table variable.
Interestingly the query plan showed the final SELECT was 1% of the total cost with a table variable and 23% with a temporary table. It just shows, you have to be careful about looking at the steps in the plan and zooming in on the step that has the largest share of the total cost. I could have gone blind looking at the first step without ever improving it.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s