SQL Server – Reducing Table Scans Using Group By and Window Functions

sql serversql-server-2016window functions

There is a code that I'm trying to improve that looks like this(simple example):

SELECT    DISTINCT a.col_a
         ,COALESCE(b1.col_c, b2.col_c, b3.col_c)
FROM      tab_a a
LEFT JOIN tab_b b1
          ON a.col_a = b1.col_a
             AND b1.col_b = 'blabla1'
LEFT JOIN tab_b b2
          ON a.col_a = b2.col_a
             AND b2.col_b = 'blabla2'
LEFT JOIN tab_b b3
          ON a.col_a = b3.col_a
             AND b3.col_b = 'blabla3';

You can use the following script to recreate those tables

CREATE TABLE tab_a(col_a int)
CREATE TABLE tab_b(col_a INT, col_b VARCHAR(10), col_c INT)

INSERT INTO dbo.tab_a ( col_a ) VALUES ( 1 ), ( 2 ), ( 3 );

INSERT INTO dbo.tab_b ( col_a
                       ,col_b
                       ,col_c )
VALUES ( 1, 'blabla1', 1 )
      ,( 1, 'blabla2', 3 )
      ,( 1, 'blabla2', 5 )
      ,( 2, 'blabla2', NULL )
      ,( 2, 'blabla3', 5 );

How can I change it to 1 join + maybe window function and how to rewrite coalesce part. Just to explain, current plan shows 3 tab_b scans, I want to reduce it to 1.

Best Answer

SELECT    DISTINCT a.col_a
         ,b.col_c
FROM      tab_a a
outer apply (select top 1 b.col_c 
             from tab_b b
             where ((a.col_a = b.col_a
                     AND b.col_b = 'blabla1' )
                  or (a.col_a = b.col_a
                     AND b.col_b = 'blabla2')
                  or (a.col_a = b.col_a
                     AND b.col_b = 'blabla3'))
                  and b.col_c is not null
            order by b.col_b)b;

This solution has 1 tab_b scan but adds sort because you want to choose b.col_c as in your COALESCE. In the example above this order corresponds to the order given by your constants in join condition that correspond to values of c column. In case when the order should be different the thing will be more complicated as you should write customized order byclause.

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