Sql-server – Index not making execution faster, and in some cases is slowing down the query. Why is it so

indexsql server

I was experimenting with indexes to speed up things, but in case of a join, the index is not improving the query execution time and in some cases it is slowing things down.

The query to create test table and fill it with data is:

CREATE TABLE [dbo].[IndexTestTable](
    [id] [int] IDENTITY(1,1) PRIMARY KEY,
    [Name] [nvarchar](20) NULL,
    [val1] [bigint] NULL,
    [val2] [bigint] NULL)

DECLARE @counter INT;
SET @counter = 1;

WHILE @counter < 500000
BEGIN
    INSERT INTO IndexTestTable
      (
        -- id -- this column value is auto-generated
        NAME,
        val1,
        val2
      )
    VALUES
      (
        'Name' + CAST((@counter % 100) AS NVARCHAR),
        RAND() * 10000,
        RAND() * 20000
      );

    SET @counter = @counter + 1;
END

-- Index in question
CREATE NONCLUSTERED INDEX [IndexA] ON [dbo].[IndexTestTable]
(
    [Name] ASC
)
INCLUDE (   [id],
    [val1],
    [val2])

Now query 1, which is improved (only slightly but the improvement is consistent) is:

SELECT *
FROM   IndexTestTable I1
       JOIN IndexTestTable I2
            ON  I1.ID = I2.ID
WHERE  I1.Name = 'Name1'

Stats and execution plan without Index (in this case the table uses the default clustered index):

(5000 row(s) affected)
Table 'IndexTestTable'. Scan count 2, logical reads 5580, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 109 ms,  elapsed time = 294 ms.

enter image description here

Now with Index enabled:

(5000 row(s) affected)
Table 'IndexTestTable'. Scan count 2, logical reads 2819, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 94 ms,  elapsed time = 231 ms.

enter image description here

Now the query which slows down due to the index (the query is meaningless since it is created for testing only):

SELECT I1.Name,
       SUM(I1.val1),
       SUM(I1.val2),
       MIN(I2.Name),
       SUM(I2.val1),
       SUM(I2.val2)
FROM   IndexTestTable I1
       JOIN IndexTestTable I2
            ON  I1.Name = I2.Name
WHERE   
       I2.Name = 'Name1'
GROUP BY
       I1.Name

With clustered index enabled:

(1 row(s) affected)
Table 'IndexTestTable'. Scan count 4, logical reads 60, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 1, logical reads 155106, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 17207 ms,  elapsed time = 17337 ms.

enter image description here

Now with Index disabled:

(1 row(s) affected)
Table 'IndexTestTable'. Scan count 5, logical reads 8642, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 2, logical reads 165212, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 17691 ms,  elapsed time = 9073 ms.

enter image description here

The questions are:

  1. Even though the index is suggested by the SQL Server, why does it
    slow things down by a significant difference?
  2. What is the Nested Loop join which is taking most of the time and
    how to improve its execution time?
  3. Is there something that I am doing wrong or have missed?
  4. With the default index (on primary key only) why does it take less
    time, and with the non clustered index present, for each row in the
    joining table, the joined table row should be found quicker, because
    join is on Name column on which the index has been created. This is
    reflected in the query execution plan and Index Seek cost is less
    when IndexA is active, but why still slower? Also what is in the
    Nested Loop left outer join that is causing the slowdown?

Using SQL Server 2012

Best Answer

Even though the index is suggested by the SQL Server, why does it slow things down by a significant difference?

Index suggestions are made by the query optimizer. If it comes across a logical selection from a table which is not well served by an existing index, it may add a "missing index" suggestion to its output. These suggestions are opportunistic; they are not based on a full analysis of the query, and do not take account of wider considerations. At best, they are an indication that more helpful indexing may be possible, and a skilled DBA should take a look.

The other thing to say about missing index suggestions is that they are based on the optimizer's costing model, and the optimizer estimates by how much the suggested index might reduce the estimated cost of the query. The key words here are "model" and "estimates". The query optimizer knows little about your hardware configuration or other system configuration options - its model is largely based on fixed numbers that happen to produce reasonable plan outcomes for most people on most systems most of the time. Aside from issues with the exact cost numbers used, the results are always estimates - and estimates can be wrong.

What is the Nested Loop join which is taking most of the time and how to improve its execution time?

There is little to be done to improve the performance of the cross join operation itself; nested loops is the only physical implementation possible for a cross join. The table spool on the inner side of the join is an optimization to avoid rescanning the inner side for each outer row. Whether this is a useful performance optimization depends on various factors, but in my tests the query is better off without it. Again, this is a consequence of using a cost model - my CPU and memory system likely has different performance characteristics than yours. There is no specific query hint to avoid the table spool, but there is an undocumented trace flag (8690) that you can use to test execution performance with and without the spool. If this were a real production system problem, the plan without the spool could be forced using a plan guide based on the plan produced with TF 8690 enabled. Using undocumented trace flags in production is not advised because the installation becomes technically unsupported and trace flags can have undesirable side-effects.

Is there something that I am doing wrong or have missed?

The main thing you are missing is that although the plan using the nonclustered index has a lower estimated cost according to the optimizer's model, it has a significant execution-time problem. If you look at the distribution of rows across threads in the plan using the Clustered Index, you will likely see a reasonably good distribution:

Scan plan

In the plan using the Nonclustered Index Seek, the work ends up being performed entirely by one thread:

Seek plan

This is a consequence of the way work is distributed among threads by parallel scan/seek operations. It is not always the case that a parallel scan will distribute work better than an index seek - but it does in this case. More complex plans might include repartitioning exchanges to redistribute work across threads. This plan has no such exchanges, so once rows are assigned to a thread, all related work is performed on that same thread. If you look at the work distribution for the other operators in the execution plan, you will see that all work is performed by the same thread as shown for the index seek.

There are no query hints to affect row distribution among threads, the important thing is to be aware of the possibility and to be able to read enough detail in the execution plan to determine when it is causing a problem.

With the default index (on primary key only) why does it take less time, and with the non clustered index present, for each row in the joining table, the joined table row should be found quicker, because join is on Name column on which the index has been created. This is reflected in the query execution plan and Index Seek cost is less when IndexA is active, but why still slower? Also what is in the Nested Loop left outer join that is causing the slowdown?

It should now be clear that the nonclustered index plan is potentially more efficient, as you would expect; it is just poor distribution of work across threads at execution time that accounts for the performance issue.

For the sake of completing the example and illustrating some of the things I have mentioned, one way to get a better work distribution is to use a temporary table to drive parallel execution:

SELECT
    val1,
    val2
INTO #Temp
FROM dbo.IndexTestTable AS ITT
WHERE Name = N'Name1';

SELECT 
    N'Name1',
    SUM(T.val1),
    SUM(T.val2),
    MIN(I2.Name),
    SUM(I2.val1),
    SUM(I2.val2)
FROM   #Temp AS T
CROSS JOIN IndexTestTable I2
WHERE
    I2.Name = 'Name1'
OPTION (FORCE ORDER, QUERYTRACEON 8690);

DROP TABLE #Temp;

This results in a plan that uses the more efficient index seeks, does not feature a table spool, and distributes work across threads well:

Optimal plan

On my system, this plan executes significantly faster than the Clustered Index Scan version.

If you're interested in learning more about the internals of parallel query execution, you might like to watch my PASS Summit 2013 session recording.

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