Table Variable Deferred Compilation: When It Works

Milos Radivojevic gives us a good example of when table variable deferred compilation is a good thing:

As mentioned in the previous article, SQL Server 2019 cardinality estimations for a table variable are based on actual table variable row counts. Therefore, in SQL Server 2019, we should expect better estimations and better plans for queries that use table variables.
Which queries will benefit from this improvement? Generally, queries that use table variables with a lot of rows in them, which are not tuned yet. For table variables with a few rows, there will not be significant changes and you should expect the same execution plan and almost same execution parameters.

Queries whose execution was slow due to underestimation in table variables usually implement logical joins by using Nested Loop Join physical operator where a Hash or Merge Join operators would be more appropriate. In addition to this, underestimation of table variables participating in multiple joins could lead to issues with insufficient memory grants, and thus data spilling to tempdb .

Click through for the example.  The next post in the series will be a case where it doesn’t work very well.

Related Posts

Calculated Columns and Memory Usage in Analysis Services

Teo Lachev troubleshoots a customer issue: Scenario: A client reports a memory spike during processing. They have a Tabular semantic model deployed to Azure Analysis Services. They fully process the model daily. The model normally takes less than 50 GB RAM but during processing, it spikes five times and Azure Analysis Services terminates the processing task […]

Read More

In the Papers: Plan Stitch

Brent Ozar reviews a Microsoft Research paper: In the Microsoft Research paper Plan Stitch: Harnessing the Best of Many Plans by Bailu Ding, Sudipto Das, Wentao Wu, Surajit Chaudhuri, and Vivek Narasayya (2018), the authors propose something really cool: watching an execution plan change over time, splitting it into parts, and then combining parts to form […]

Read More

Categories

October 2018
MTWTFSS
« Sep Nov »
1234567
891011121314
15161718192021
22232425262728
293031