SQL Server使用order by子句可显着提高选择性能

Question

I am executing the following query directly in SQL Server:

   SELECT *
     FROM TableA
LEFT JOIN TableB 
       ON TableB.field1 = TableA.field1
LEFT JOIN TableC 
       ON TableC.field2 = TableA.field2
LEFT JOIN TableD 
       ON TableD.field3 = TableA.field3
LEFT JOIN TableE 
       ON TableE.field4 = TableA.field4
LEFT JOIN TableF 
       ON TableF.field5 = TableA.field5
LEFT JOIN 
          (SELECT *
             FROM 
                (SELECT 
                       Id1, Id2,
                       UpdateDate,
                       ROW_NUMBER() OVER(PARTITION BY Id1, Id2, 
               ORDER BY UpdateDate DESC) AS RN 
                   FROM TableG) AS G 
           WHERE G.RN = 1) TableH 
     ON TableA.Id1 = TableH.Id2 
    AND TableA.Id1 = TableH.Id2

For point of reference, Table AF and G are about 1000 rows, and Table G is about 10000 rows.

For a particular input, this query takes about 1 minute to run.

I then add a

ORDER BY Id1 ASC

at the end of the statement, and now it takes about 6 seconds to run. How can adding a sort significantly improve performance like this?

Answer 1

Run a showplan on both versions of your query.

Probably what's happening is the sort forces a different query plan, which uses a more efficient for your particular data join strategy (probably in-memory), but which has a higher estimated cost.

Answer 2

After examining the execution plan, it seems that the issue was with the JOIN on Table A and Table G. Initially the optimizer was trying to use a nested loop join which was very inefficient for tables of their size. Adding the ORDER BY clause hinted to the optimizer to use a merge join instead, which was much faster. Thanks for the answers!