Deeper Into Adaptive Join Optimization

Erik Darling has a couple blog posts getting deeper into Adaptive Join Optimizations in SQL Server 2017.  First, Erik discusses the basics:

You see, in every plan, you see both possible paths the optimizer could have chosen. Right now it’s only limited to two choices, Nested Loops and Hash Joins.

Just guessing that Merge Joins weren’t added because there would have been additional considerations around the potential cost of a Sort operation to get the data in order.

Be sure to read Brent’s comment that in the initial release, it will just support columnstore indexes.  Then, Erik talks about execution plan details:

Some points of interest:

  • Actual Join Type: doesn’t tell you whether it chose Hash or Nested Loops
  • Estimated Join Type: Probably does
  • Adaptive Threshold Rows: If the number of rows crosses this boundary, Join choice will change. Over will be Hash, under will be Nested Loops.

The rest is fairly self-explanatory and is the usual stuff in query plans.

Good stuff here.

Related Posts

Units Of Measure In The ShowPlan Schema

Grant Fritchey shows off the ShowPlan Schema: Because the showplan schema contains notes throughout stating what the units of measure are, what each of the values means. For instance, I can explain why SerialDesiredMemory, DesiredMemory, RequestedMemory are identical: …Provide memory grant estimate as well as actual runtime memory grant information. Serial required/desired memory attributes are […]

Read More

When AT TIME ZONE Is Too Slow

Robert Davis troubleshoots a performance problem relating to time zones: Time Zones were definitely being a drag today. I got an email from one of the developers at work asking about the performance difference between 2 queries. The only difference between the 2 queries is that one of them uses the AT TIME ZONE clause that was […]

Read More

Categories