Adaptive Join Behavior

Grant Fritchey explains adaptive joins in SQL Server 2017:

Currently the adaptive join only works with columnstore indexes, but according to Microsoft, at some point, they will also work with rowstore. The concept is simple. For larger data sets, frequently (but not always, let’s not try to cover every possible caveat, it depends, right), a hash join is much faster than a loops join. For smaller data sets, frequently, a loops join is faster. Wouldn’t it be nice if we could change the join type, on the fly, so that the most effective join was used depending on the data in the query. Ta-da, enter the adaptive join.

First, the statistics are used at compile time for the tables we’re joining. Based on those statistics, a row target it set. Below that threshold, a loops join will be used. Above that threshold, a hash join. The way the row count is determined is that the operator will always build the hash table. With the hash table built and loaded, it will know how many rows it has. If it’s going to do a loops join, the hash table is tossed and a loops join commences. If the threshold has been passed on the row counts and it’s going to do a hash join, it already has the hash table built and proceeds to do a hash join. It’s easy to understand. Let’s see it in action.

Click through to see it in action.  It’s not a world-changer yet, but as it becomes available to rowstore queries (without the filtered, empty columnstore index trick), I think people will come to appreciate it.

Related Posts

When Wait Stats Aren’t Enough

Joe Obbish has an example of diagnosing performance problems when wait stats don’t indicate any problems: In summary, page allocations and page free events rapidly occur, sometimes in an alternating pattern. SQL Server will often free a number of pages just to immediately request allocations for a similar number of pages. If all of the […]

Read More

Nested Loops, Hash, Or Merge: Which Is Best?

Grant Fritchey dodges the important questions: First response, also a joke, was the question at the title of this post: What is the preferred operator when joining tables: Hash Match, Nested Loops or Merge? While my immediate response to this question is, yes. Meaning, they’re all preferred, situationally. I decided to expand on that a […]

Read More

Categories

February 2018
MTWTFSS
« Jan Mar »
 1234
567891011
12131415161718
19202122232425
262728