How can I get rid of an unhelpful parallel branch when unpivoting a single row?





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3















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:



round robin



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:



enter image description here



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;









share|improve this question





























    3















    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:



    round robin



    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:



    enter image description here



    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;









    share|improve this question

























      3












      3








      3








      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:



      round robin



      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:



      enter image description here



      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;









      share|improve this question














      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:



      round robin



      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:



      enter image description here



      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;






      sql-server sql-server-2017






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 2 hours ago









      Joe ObbishJoe Obbish

      22.3k43493




      22.3k43493






















          1 Answer
          1






          active

          oldest

          votes


















          3














          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:



          enter image description here



          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.






          share|improve this answer
























          • I love APPLY versus UNPIVOT, but one would expect the declarative nature of T-SQL to result in the same plan shape for both variants. It'll be interesting to see what happens with this.

            – Max Vernon
            30 mins ago












          Your Answer








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          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          3














          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:



          enter image description here



          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.






          share|improve this answer
























          • I love APPLY versus UNPIVOT, but one would expect the declarative nature of T-SQL to result in the same plan shape for both variants. It'll be interesting to see what happens with this.

            – Max Vernon
            30 mins ago
















          3














          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:



          enter image description here



          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.






          share|improve this answer
























          • I love APPLY versus UNPIVOT, but one would expect the declarative nature of T-SQL to result in the same plan shape for both variants. It'll be interesting to see what happens with this.

            – Max Vernon
            30 mins ago














          3












          3








          3







          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:



          enter image description here



          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.






          share|improve this answer













          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:



          enter image description here



          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.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 2 hours ago









          Joe ObbishJoe Obbish

          22.3k43493




          22.3k43493













          • I love APPLY versus UNPIVOT, but one would expect the declarative nature of T-SQL to result in the same plan shape for both variants. It'll be interesting to see what happens with this.

            – Max Vernon
            30 mins ago



















          • I love APPLY versus UNPIVOT, but one would expect the declarative nature of T-SQL to result in the same plan shape for both variants. It'll be interesting to see what happens with this.

            – Max Vernon
            30 mins ago

















          I love APPLY versus UNPIVOT, but one would expect the declarative nature of T-SQL to result in the same plan shape for both variants. It'll be interesting to see what happens with this.

          – Max Vernon
          30 mins ago





          I love APPLY versus UNPIVOT, but one would expect the declarative nature of T-SQL to result in the same plan shape for both variants. It'll be interesting to see what happens with this.

          – Max Vernon
          30 mins ago


















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