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Consider the following query that unpivots a few handfuls of scalar aggregates:

SELECT A, B
FROM (
 SELECT 
 MAX(CASE WHEN ID = 1 THEN 1 ELSE 0 END) VAL1
 , MAX(CASE WHEN ID = 2 THEN 1 ELSE 0 END) VAL2
 , MAX(CASE WHEN ID = 3 THEN 1 ELSE 0 END) VAL3
 , MAX(CASE WHEN ID = 4 THEN 1 ELSE 0 END) VAL4
 , MAX(CASE WHEN ID = 5 THEN 1 ELSE 0 END) VAL5
 , MAX(CASE WHEN ID = 6 THEN 1 ELSE 0 END) VAL6
 , MAX(CASE WHEN ID = 7 THEN 1 ELSE 0 END) VAL7
 , MAX(CASE WHEN ID = 16 THEN 1 ELSE 0 END) VAL16
 FROM dbo.PARALLEL_ZONE_REPRO
) q
UNPIVOT(B FOR A IN (
 VAL1
 ,VAL2
 ,VAL3
 ,VAL4
 ,VAL5
 ,VAL6
 ,VAL7
 ,VAL16
)) U
OPTION (MAXDOP 4);

On SQL Server 2017, I get a plan with two parallel branches. The left parallel branch feels out of place to me. The optimizer has a guarantee that there will be only a single row output from the global scalar aggregate, yet the parent operator of it is a Distribute Streams with round robin partitioning:

When I execute the query all of the rows go to a single thread as expected. There's no performance problem with this query, but the query reserves 8 parallel threads with MAXDOP set to 4. Again, I feel that this is out of place. It's impossible for both parallel branches to execute at the same time. I want to avoid unnecessary worker thread reservation because I have TF 2467 enabled which changes the scheduling algorithm to look at the number of worker threads per scheduler.

Is it possible to rewrite the query to have exactly one parallel branch that contains the table scan and local aggregate? For example, I would be fine with the general shape below except that I want the nested loop to execute in a serial zone:

For Application Reasons™ I strongly prefer to avoid splitting this query up into parts. If desired, you can view the actual query plan here. If you'd like to play along at home, here is T-SQL to create the table used in the query:

DROP TABLE IF EXISTS dbo.PARALLEL_ZONE_REPRO;

CREATE TABLE dbo.PARALLEL_ZONE_REPRO (
 ID BIGINT,
 FILLER VARCHAR(100)
);

INSERT INTO dbo.PARALLEL_ZONE_REPRO WITH (TABLOCK)
SELECT
 ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) % 15
, REPLICATE('Z', 100)
FROM master..spt_values t1
CROSS JOIN master..spt_values t2;

I am able to get the desired plan shape with a serial loop join when all of the following are true:

• An APPLY or CROSS JOIN is used instead of UNPIVOT
  


• The APPLY contains no outer references


• The source of rows in the APPLY is a table value constructor as opposed to a table

For example, here is one way to do it:

SELECT A, B
FROM 
(
 SELECT A
 , MAX(
 CASE
 WHEN A = 'VAL1' THEN VAL1 
 WHEN A = 'VAL2' THEN VAL2
 WHEN A = 'VAL3' THEN VAL3
 WHEN A = 'VAL4' THEN VAL4
 WHEN A = 'VAL5' THEN VAL5
 WHEN A = 'VAL6' THEN VAL6
 WHEN A = 'VAL7' THEN VAL7
 WHEN A = 'VAL16' THEN VAL16
 ELSE NULL
 END
 ) B
 FROM (
 SELECT 
 MAX(CASE WHEN ID = 1 THEN 1 ELSE 0 END) VAL1
 , MAX(CASE WHEN ID = 2 THEN 1 ELSE 0 END) VAL2
 , MAX(CASE WHEN ID = 3 THEN 1 ELSE 0 END) VAL3
 , MAX(CASE WHEN ID = 4 THEN 1 ELSE 0 END) VAL4
 , MAX(CASE WHEN ID = 5 THEN 1 ELSE 0 END) VAL5
 , MAX(CASE WHEN ID = 6 THEN 1 ELSE 0 END) VAL6
 , MAX(CASE WHEN ID = 7 THEN 1 ELSE 0 END) VAL7
 , MAX(CASE WHEN ID = 16 THEN 1 ELSE 0 END) VAL16
 FROM dbo.PARALLEL_ZONE_REPRO
 ) q
 CROSS APPLY (
 VALUES ('VAL1'), ('VAL2'), ('VAL3'), ('VAL4'),
 ('VAL5'), ('VAL6'), ('VAL7'), ('VAL16') 
 ) ca (A)
 GROUP BY A
) q
WHERE q.B IS NOT NULL
OPTION (MAXDOP 4);

I get the desired plan plan shape as claimed with just one parallel branch:

I tried many other things that did not work. This answer is unsatisfactory in that I don't know why it works and it may not work in a future version of SQL Server, but it did solve my problem.