SQL Server 2017: How do Batch Mode Adaptive Joins work?

Batch Mode Adaptive Joins are part of a family of features in the 2017 query processor that consists of:

• Batch Mode Adaptive Joins

• Interleaved Execution

• Batch Mode Memory Grant Feedback

So how do Adaptive Joins work?

Batch Mode Adaptive Joins

For Batch Mode Adaptive Joins, the goal is to not pin join choice to a specific type at compile time.

When available, Adaptive Joins allow the optimizer to choose between Nested Loops Joins and Hash Joins based on row thresholds at run time.

At this time, Merge Joins are not considered. Pure speculation is that needing data to be sorted, or needing to inject a sort into the plan would add too much overhead when changing the course of a query.

When Do Batch Mode Adaptive Joins Occur?

At this time, Batch Mode query processing requires the presence of a ColumnStore index. They also require, well, a join, and an index that allows for the choice of a Nested Loops or Hash Join.

How do I know if my Join is Adaptive?

Query plans for adaptive joins are quite distinctive.

The Adaptive Join operator is new to SQL Server 2017, and currently has the following properties in actual execution plans.

-
Physical Operation: Adaptive Join

-
Actual Join Type: Will be Hash match or Nested Loops

-
Adaptive Threshold Rows: Signifies the tipping point when the join type will switch to Hash Match

-
Is Adaptive: True for Adaptive Joins

-
Estimated Join Type: Rather self-explanatory!

In an estimated or cached plan, there is rather less information:

Most notably, the Actual Join Type is missing.

What breaks Batch Mode Adaptive Joins?

To monitor this, there's an Extended Event session called adaptive_join_skipped, and it has the following reasons for skipping a Batch Mode Adaptive Join:

• eajsrExchangeTypeNotSupported

• eajsrHJorNLJNotFound

• eajsrInvalidAdaptiveThreshold

• eajsrMultiConsumerSpool

• eajsrOuterCardMaxOne

• eajsrOuterSideParallelMarked

• eajsrUnMatchedOuter

Aside from those, Batch Mode Adaptive Joins may be skipped for other reasons. Take these two queries for example:

/*Selecting just integer data*/
SELECT uc.Id, uc.Reputation, p.Score
FROM   dbo.Users_cx AS uc
JOIN   dbo.Posts AS p
ON p.OwnerUserId = uc.Id
WHERE  uc.LastAccessDate >= '20160101';

/*Selecting one string column from Users*/
SELECT uc.Id, uc.DisplayName, uc.Reputation, p.Score
FROM   dbo.Users_cx AS uc
JOIN   dbo.Posts AS p
ON p.OwnerUserId = uc.Id
WHERE  uc.LastAccessDate >= '20160101';

They're identical except that the second query selects the DisplayName column, which has a type of NVARCHAR(40).

The Batch Mode Adaptive Join is skipped for the second query, but no reason is logged to the XE session. It appears that string data remains the steadfast enemy of ColumnStore indexes.

There are other query patterns that fail to get Adaptive Joins, that also do not trigger events.

Some examples:

-
CROSS APPLY with a TOP

-
OUTER APPLY

eajsrExchangeTypeNotSupported

One thing that will trigger this event appears to be the presence of a Repartition Streams operator. In this query, the partitioning type is Hash Match. Special thanks to the Intergalactic Celestial Being masquerading as a humble blogging man known as Paul White for the bizarre query.

SELECT uc.Id, uc.Reputation, CONVERT(XML, CONVERT(NVARCHAR(10), p.Score)).value('(xml/text())[1]', 'INT') AS [Surprise!]
FROM   dbo.Users_cx AS uc
JOIN   dbo.Posts AS p
ON p.OwnerUserId = uc.Id
WHERE  uc.LastAccessDate <= '2009-08-01'
OPTION ( USE HINT ( 'ENABLE_PARALLEL_PLAN_PREFERENCE' ));
GO

eajsrHJorNLJNotFound

No queries have triggered this XE yet.
What doesn't work:

-
Merge Join hint

-
Query patterns that rule out join type, for example Hash and Merge joins require at least one equality predicate. Writing a join on >= and

eajsrInvalidAdaptiveThreshold

This event can be triggered by various TOP, FAST N, and OFFSET/FETCH queries. Here are some examples:

SELECT uc.Id, uc.DisplayName, uc.Reputation, p.Score
FROM   dbo.Users_cx AS uc
INNER JOIN   dbo.Posts AS p
ON p.OwnerUserId = uc.Id
WHERE  uc.LastAccessDate <= '2008-01-01'
OPTION ( FAST 1);
GO 

SELECT TOP 1 uc.Id, uc.DisplayName, uc.Reputation, p.Score
FROM   dbo.Users_cx AS uc
INNER JOIN   dbo.Posts AS p
ON p.OwnerUserId = uc.Id
WHERE  uc.LastAccessDate <= '2008-01-01';
GO

In some circumstances, they can also be triggered by paging-style queries:

WITH pages
AS ( SELECT TOP 100 uc.Id, ROW_NUMBER() OVER ( ORDER BY uc.Id ) AS n
     FROM   dbo.Users_cx AS uc ),
     rows
AS ( SELECT TOP 50 p.Id
     FROM   pages AS p
     WHERE  p.n > 50 )
SELECT u.Id, u.Reputation
FROM   pages AS p
JOIN   dbo.Users AS u
ON p.Id = u.Id;

eajsrMultiConsumerSpool

No known query patterns have triggered this event yet.

What hasn't triggered it so far:

-
Recursive CTEs

-
Grouping sets/Cube/Rollup

-
PIVOT and UNPIVOT

-
Windowing functions

eajsrOuterCardMaxOne

A couple different types of queries have triggered this event. A derived join with a TOP 1, and a join combined with a WHERE clause with an equality predicate on a unique column:

``
SELECT uc.Id, uc.Reputation, p.Score
FROM dbo.Users_cx AS uc
JOIN (SELECT TOP 1 p2.OwnerUserId, p2.Score F

StackExchange Database Administrators Q#187576, answer score: 12