How to improve the performance of a SQL Server Select query?

Question

The following T-SQL query is taking 54 seconds to execute:

SELECT top (3)
   a.c1, b.c2, c.c3, d.c4 
FROM 
   table1 as a WITH (NOLOCK) 
JOIN 
   table2 as b WITH (NOLOCK) ON a.c1 = b.c4 
LEFT JOIN 
   table3 as c WITH (NOLOCK) ON a.c1 = c.c4 
LEFT JOIN 
   table4 as d WITH (NOLOCK) ON b.c3 = d.c1 
WHERE 
   b.source = '8R' 
ORDER BY 
   b.RecvdDate ASC

Whereas the same query sorted in descending order execute within a second

SELECT top (3)
   a.c1, b.c2, c.c3, d.c4 
FROM 
   table1 as a WITH (NOLOCK) 
JOIN 
   table2 as b WITH (NOLOCK) ON a.c1 = b.c4 
LEFT JOIN 
   table3 as c WITH (NOLOCK) ON a.c1 = c.c4 
LEFT JOIN 
   table4 as d WITH (NOLOCK) ON b.c3 = d.c1 
WHERE 
   b.source = '8R' 
ORDER BY 
   b.RecvdDate DESC

So how can I improve the performance of the query for both the sort orders?

I am using Microsoft SQL Server 2012 (SP1) - 11.0.3128.0 (X64) Standard Edition (64-bit) on Windows NT 6.2 (Build 9200: )

Execution Plan using SET STATISTICS PROFILE ON as follows:

Rows                 Executes             StmtText                                                                                                                                                                                                                                                         StmtId      NodeId      Parent      PhysicalOp                     LogicalOp                      Argument                                                                                                                                                                                                                                                         DefinedValues                                                                         EstimateRows  EstimateIO    EstimateCPU   AvgRowSize  TotalSubtreeCost OutputList                                                                                                                       Warnings                        Type                                                             Parallel EstimateExecutions
-------------------- -------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ----------- ----------- ----------- ------------------------------ ------------------------------ ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------- ------------- ------------- ------------- ----------- ---------------- -------------------------------------------------------------------------------------------------------------------------------- ------------------------------- ---------------------------------------------------------------- -------- ------------------
3                    1                    SELECT top 3 a.c1, b.c2, c.c3, d.c4  
FROM table1 as a WITH (NOLOCK) JOIN table2 as b WITH (NOLOCK) ON a.c1 = b.c1 LEFT JOIN table3 as c WITH (NOLOCK) ON 1           1           0           NULL                           NULL                           NULL                                                                                                                                                                                                                                                             NULL                                                                                  3             NULL          NULL          NULL        0.1093194        NULL                                                                                                                             NULL                            SELECT                                                           0        NULL
3                    1                      |--Top(TOP EXPRESSION:((3)))                                                                                                                                                                                                                                   1           2           1           Top                            Top                            TOP EXPRESSION:((3))                                                                                                                                                                                                                                             NULL                                                                                  3             0             3E-07         93          0.1093194        [a].[c1], [table2].[c2], [c].[c3], [d].[c4]                                                     NULL                            PLAN_ROW                                                         0        1
3                    1                           |--Nested Loops(Left Outer Join, WHERE:([DB].[dbo].[table3].[c3] as [table3].[c3]=[DB].[dbo].[table4].[c1] as [d].[c1]))                                                                                1           3           2           Nested Loops                   Left Outer Join                WHERE:([DB].[dbo].[table2].[c3] as [table2].[c3]=[DB].[dbo].[table4].[id] as [d].[id])                                                                                                                         NULL                                                                                  3             0             221.8893      101         0.1093191        [a].[c1], [table2].[c2], [table2].[RecvdDate], [c].[c3], [d].[c4]               NULL                            PLAN_ROW                                                         0        1
3                    1                                |--Nested Loops(Left Outer Join, OUTER REFERENCES:([a].[c1], [Expr1012]) WITH ORDERED PREFETCH)                                                                                                                                          1           4           3           Nested Loops                   Left Outer Join                OUTER REFERENCES:([a].[c1], [Expr1012]) WITH ORDERED PREFETCH                                                                                                                                                                                        NULL                                                                                  3             0             18.49077      99          0.1057252        [a].[c1], [table2].[c2], [table2].[RecvdDate], [table2].[C3], [c].[c3]  NULL                            PLAN_ROW                                                         0        1
3                    1                                |    |--Nested Loops(Inner Join, OUTER REFERENCES:([table2].[c4], [Expr1011]) WITH ORDERED PREFETCH)                                                                                                                                  1           6           4           Nested Loops                   Inner Join                     OUTER REFERENCES:([table2].[c4], [Expr1011]) WITH ORDERED PREFETCH                                                                                                                                                                                NULL                                                                                  3             0             18.78465      31          0.09258068       [a].[c1], [table2].[c2], [table2].[RecvdDate], [table2].[C3]               NULL                            PLAN_ROW                                                         0        1
27                   1                                |    |    |--Nested Loops(Inner Join, OUTER REFERENCES:([table2].[c1], [Expr1010]) WITH ORDERED PREFETCH)                                                                                                                            1           8           6           Nested Loops                   Inner Join                     OUTER REFERENCES:([table2].[c1], [Expr1010]) WITH ORDERED PREFETCH                                                                                                                                                                               NULL                                                                                  22.09678      0             136.1955      41          0.07926979       [table2].[c4], [table2].[c2], [table2].[RecvdDate], [table2].[C3]       NULL                            PLAN_ROW                                                         0        1
19928681             1                                |    |    |    |--Index Scan(OBJECT:([DB].[dbo].[table2].[IX_datereceived] AS [table2]), ORDERED BACKWARD)                                                                                                     1           10          8           Index Scan                     Index Scan                     OBJECT:([DB].[dbo].[table2].[IX_datereceived] AS [table2]), ORDERED BACKWARD                                                                                                                                               [table2].[c1], [table2].[RecvdDate]                   22.09678      66.78609      35.84109      19          0.003349548      [table2].[c1], [table2].[RecvdDate]                                                              NULL                            PLAN_ROW                                                         0        1
27                   19928681                         |    |    |    |--Clustered Index Seek(OBJECT:([DB].[dbo].[table2].[PK_c1] AS [table2]), SEEK:([table2].[c1]=[DB].[dbo].[table2].[c1] as [table2].[c1]),   1           12          8           Clustered Index Seek           Clustered Index Seek           OBJECT:([DB].[dbo].[table2].[PK_c1] AS [table2]), SEEK:([table2].[c1]=[DB].[dbo].[table2].[c1] as [table2].[c1]),  WHERE:([DB].[dbo].[table2].[Source]  [table2].[c4], [table2].[c2], [table2].[C3]  3.047679      0.003125      0.0001581     77          0.07582787       [table2].[c4], [table2].[c2], [table2].[C3]                                             NULL                            PLAN_ROW                                                         0        23.09678
3                    3                                |    |    |--Index Seek(OBJECT:([DB].[dbo].[table1].[PK_c1] AS [a]), SEEK:([a].[c1]=[DB].[dbo].[table2].[c4] as [table2].[c4]) ORDERED FORWARD)                                  1           20          6           Index Seek                     Index Seek                     OBJECT:([DB].[dbo].[table1].[PK_c1] AS [a]), SEEK:([a].[c1]=[DB].[dbo].[table2].[c4] as [table2].[c4]) ORDERED FORWARD                                                                       [a].[c1]                                                                  1             0.003125      0.0001581     11          0.02297952       [a].[c1]                                                                                                             NULL                            PLAN_ROW                                                         0        7.000085
3                    3                                |    |--Clustered Index Seek(OBJECT:([DB].[dbo].[table3].[table3_PK] AS [c]), SEEK:([c].[c4]=[DB].[dbo].[table1].[c1] as [a].[c1]) ORDERED FORWARD)                                    1           21          4           Clustered Index Seek           Clustered Index Seek           OBJECT:([DB].[dbo].[table3].[table3_PK] AS [c]), SEEK:([c].[c4]=[DB].[dbo].[table1].[c1] as [a].[c1]) ORDERED FORWARD                                                                              [c].[c3]                                                                           1             0.003125      0.0001581     75          0.01313197       [c].[c3]                                                                                                                      NULL                            PLAN_ROW                                                         0        4
36                   3                                |--Table Scan(OBJECT:([DB].[dbo].[table4] AS [d]))                                                                                                                                                                                    1           22          3           Table Scan                     Table Scan                     OBJECT:([DB].[dbo].[table4] AS [d])                                                                                                                                                                                                               [d].[id], [d].[c4]                                                            12            0.0032035     9.17E-05      17          0.0035703        [d].[id], [d].[c4]                                                                                                       NO STATS:([table4].[c1])  PLAN_ROW                                                         0        3.999999

(11 row(s) affected)

Link to execution plan in xml format:

Answer 1

table2 has 32,582,700 rows.

The estimated plan looks as follows.

You have a non covering index on RecvdDate and the plan has key lookups to retrieve the missing columns and evaluate the predicate on source = '8R'.

SQL Server estimates that it will need to do 22 -23 such lookups before it finds the TOP 3 matching rows and it can exit.

This assumes that rows matching the 8R predicate are scattered evenly with respect to date. In your case they are not and they are all on later dates. It it is actually doing 19,928,681 lookups in reality (from STATISTICS PROFILE output in question) rather than the 20 or so estimated.

The easiest way to fix it will be to supply an index on source, RecvdDate. As far as this query goes it doesn't matter what direction. Any of the four possibilities would work.

source asc, RecvdDate asc
source asc, RecvdDate desc
source desc, RecvdDate asc
source desc, RecvdDate desc

It can still do an equality seek on source and traverse the matching rows forward or backwards as required to get RecvdDate ASC or RecvdDate DESC.

Answer 2

Is that performance consistent? With that big a different my guess is that you have a composite index on

source, RecvdDate DESC

which means that it can't be used when querying by RecvdDate in ascending order.

You could add separate indices for source and RecvdDate which would improve performance (with a single column index the order does not matter), or add another composite index with the RecvdDate in ascending order.