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Deeper insight into unused indexes for SQL Server

By: Greg Robidoux | Read Comments (12) | Related Tips: More > Indexing

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Problem
One of the balancing acts of SQL Server is the use of indexes. Too few indexes can cause scans to occur which hurts performance and too many indexes causes overhead for index maintenance during data updates and also a bloated database. So what steps can be taken to determine which indexes are being used and how they are being used.

Solution
In a previous tip, How to get index usage information in SQL Server, we talked about how to get index usage information by using the DMVs sys.dm_db_index_operational_stats and sys.dm_db_index_usage_stats. The one issue with these views is that it only gives you part of the picture and you still need to dig deeper to get all of the information you need to determine which indexes are not used.

In this tip we will take it a step further and provide some additional queries that will shed some much needed light on the issue.

We will be using the DMV sys.dm_db_index_usage_stats which keeps track of each index that has been used and how it has been used. This was covered in the tip mentioned above. These stats are collected whenever an object is accessed. SQL Server resets these values if SQL Server is restarted or if you detach and reattach the database.

There are seven queries below and each one builds upon the others to give you more and more information. If you can't wait you can just skip right to Query 7. To select the code you should be able to triple click in the table cell to select all of the code.

Note: the information below was collected by restarting SQL Server and then doing selected queries on the AdventureWorks database. For a more heavily used database your numbers and index usage will be much higher.

Query 1

In this first query we are just using sys.dm_db_index_usage_stats and sys.objects to get a list of the indexes that have been used and how they are being used.

SELECT DB_NAME(DATABASE_ID) AS DATABASENAME, 
       OBJECT_NAME(B.OBJECT_ID) AS TABLENAME, 
       INDEX_NAME = (SELECT NAME 
                     FROM   SYS.INDEXES A 
                     WHERE  A.OBJECT_ID = B.OBJECT_ID 
                            AND A.INDEX_ID = B.INDEX_ID), 
       USER_SEEKS, 
       USER_SCANS, 
       USER_LOOKUPS, 
       USER_UPDATES 
FROM   SYS.DM_DB_INDEX_USAGE_STATS B 
       INNER JOIN SYS.OBJECTS C 
         ON B.OBJECT_ID = C.OBJECT_ID 
WHERE  DATABASE_ID = DB_ID(DB_NAME()) 
       AND C.TYPE <> 'S'

Query 2

In this query we are listing each user table and all of the tables indexes that have not been used by using a NOT EXISTS against sys.dm_db_index_usage_stats.

SELECT   DB_NAME() AS DATABASENAME, 
         OBJECT_NAME(B.OBJECT_ID) AS TABLENAME, 
         B.NAME AS INDEXNAME, 
         B.INDEX_ID 
FROM     SYS.OBJECTS A 
         INNER JOIN SYS.INDEXES B 
           ON A.OBJECT_ID = B.OBJECT_ID 
WHERE    NOT EXISTS (SELECT * 
                     FROM   SYS.DM_DB_INDEX_USAGE_STATS C 
                     WHERE  B.OBJECT_ID = C.OBJECT_ID 
                            AND B.INDEX_ID = C.INDEX_ID) 
         AND A.TYPE <> 'S' 
ORDER BY 1, 2, 3

Query 3

In this query we are listing each user table, all of its indexes and the columns that make up the index. The issue with this query is that you have a row for each column in the index which could get confusing if you have a lot of indexes.

SELECT   A.NAME, 
         B.NAME, 
         C.KEY_ORDINAL, 
         D.NAME 
FROM     SYS.OBJECTS A 
         INNER JOIN SYS.INDEXES B 
           ON A.OBJECT_ID = B.OBJECT_ID 
         INNER JOIN SYS.INDEX_COLUMNS C 
           ON B.OBJECT_ID = C.OBJECT_ID 
              AND B.INDEX_ID = C.INDEX_ID 
         INNER JOIN SYS.COLUMNS D 
           ON C.OBJECT_ID = D.OBJECT_ID 
              AND C.COLUMN_ID = D.COLUMN_ID 
WHERE    A.TYPE <> 'S' 
ORDER BY 1, 2, 3

Query 4

In this query we use most of Query 3, but we are doing a PIVOT so we can see the index and the index columns in one row. This only accounts for 7 index columns, but it could easily be increased to handle more in the PIVOT operation. Here is another tip related to the use of PIVOT, Crosstab queries using PIVOT in SQL Server 2005. if you would like to better understand how PIVOT works.

SELECT   TABLENAME, INDEXNAME, INDEXID, [1] AS COL1, [2] AS COL2, [3] AS COL3, 
         [4] AS COL4,  [5] AS COL5, [6] AS COL6, [7] AS COL7 
FROM     (SELECT A.NAME AS TABLENAME, 
                 B.NAME AS INDEXNAME, 
                 B.INDEX_ID AS INDEXID, 
                 D.NAME AS COLUMNNAME, 
                 C.KEY_ORDINAL 
          FROM   SYS.OBJECTS A 
                 INNER JOIN SYS.INDEXES B 
                   ON A.OBJECT_ID = B.OBJECT_ID 
                 INNER JOIN SYS.INDEX_COLUMNS C 
                   ON B.OBJECT_ID = C.OBJECT_ID 
                      AND B.INDEX_ID = C.INDEX_ID 
                 INNER JOIN SYS.COLUMNS D 
                   ON C.OBJECT_ID = D.OBJECT_ID 
                      AND C.COLUMN_ID = D.COLUMN_ID 
          WHERE  A.TYPE <> 'S') P 
         PIVOT 
         (MIN(COLUMNNAME) 
          FOR KEY_ORDINAL IN ( [1],[2],[3],[4],[5],[6],[7] ) ) AS PVT 
ORDER BY TABLENAME, INDEXNAME;

Query 5

In this query we tie in our PIVOT query above with sys.dm_db_index_usage_stats so we can look at only the indexes that have been used since the last time the stats were reset.

SELECT   TABLENAME, INDEXNAME, INDEX_ID, [1] AS COL1, [2] AS COL2, [3] AS COL3, 
         [4] AS COL4, [5] AS COL5, [6] AS COL6, [7] AS COL7 
FROM     (SELECT A.NAME AS TABLENAME, 
                 A.OBJECT_ID, 
                 B.NAME AS INDEXNAME, 
                 B.INDEX_ID, 
                 D.NAME AS COLUMNNAME, 
                 C.KEY_ORDINAL 
          FROM   SYS.OBJECTS A 
                 INNER JOIN SYS.INDEXES B 
                   ON A.OBJECT_ID = B.OBJECT_ID 
                 INNER JOIN SYS.INDEX_COLUMNS C 
                   ON B.OBJECT_ID = C.OBJECT_ID 
                      AND B.INDEX_ID = C.INDEX_ID 
                 INNER JOIN SYS.COLUMNS D 
                   ON C.OBJECT_ID = D.OBJECT_ID 
                      AND C.COLUMN_ID = D.COLUMN_ID 
          WHERE  A.TYPE <> 'S') P 
         PIVOT 
         (MIN(COLUMNNAME) 
          FOR KEY_ORDINAL IN ( [1],[2],[3],[4],[5],[6],[7] ) ) AS PVT 
WHERE    EXISTS (SELECT OBJECT_ID, 
                        INDEX_ID 
                 FROM   SYS.DM_DB_INDEX_USAGE_STATS B 
                 WHERE  DATABASE_ID = DB_ID(DB_NAME()) 
                        AND PVT.OBJECT_ID = B.OBJECT_ID 
                        AND PVT.INDEX_ID = B.INDEX_ID) 
ORDER BY TABLENAME, INDEXNAME;

Query 6

This query also uses the PIVOT query along with sys.dm_db_index_usage_stats so we can also see the stats on the indexes that have been used.

SELECT   PVT.TABLENAME, PVT.INDEXNAME, [1] AS COL1, [2] AS COL2, [3] AS COL3, 
         [4] AS COL4, [5] AS COL5, [6] AS COL6, [7] AS COL7, B.USER_SEEKS, 
         B.USER_SCANS, B.USER_LOOKUPS 
FROM     (SELECT A.NAME AS TABLENAME, 
                 A.OBJECT_ID, 
                 B.NAME AS INDEXNAME, 
                 B.INDEX_ID, 
                 D.NAME AS COLUMNNAME, 
                 C.KEY_ORDINAL 
          FROM   SYS.OBJECTS A 
                 INNER JOIN SYS.INDEXES B 
                   ON A.OBJECT_ID = B.OBJECT_ID 
                 INNER JOIN SYS.INDEX_COLUMNS C 
                   ON B.OBJECT_ID = C.OBJECT_ID 
                      AND B.INDEX_ID = C.INDEX_ID 
                 INNER JOIN SYS.COLUMNS D 
                   ON C.OBJECT_ID = D.OBJECT_ID 
                      AND C.COLUMN_ID = D.COLUMN_ID 
          WHERE  A.TYPE <> 'S') P 
         PIVOT 
         (MIN(COLUMNNAME) 
          FOR KEY_ORDINAL IN ( [1],[2],[3],[4],[5],[6],[7] ) ) AS PVT 
         INNER JOIN SYS.DM_DB_INDEX_USAGE_STATS B 
           ON PVT.OBJECT_ID = B.OBJECT_ID 
              AND PVT.INDEX_ID = B.INDEX_ID 
              AND B.DATABASE_ID = DB_ID() 
ORDER BY TABLENAME, INDEXNAME;

Query 7

This last query allow us to see both used and unused indexes. Since the DMV sys.dm_db_index_usage_stats only tracks when an index is used it is hard to compare the used and unused indexes. The query below allows you to see all indexes to compare both used and unused indexes since the stats were collected by using a UNION.

SELECT PVT.TABLENAME, PVT.INDEXNAME, PVT.INDEX_ID, [1] AS COL1, [2] AS COL2, [3] AS COL3, 
       [4] AS COL4,  [5] AS COL5, [6] AS COL6, [7] AS COL7, B.USER_SEEKS, 
       B.USER_SCANS, B.USER_LOOKUPS 
FROM   (SELECT A.NAME AS TABLENAME, 
               A.OBJECT_ID, 
               B.NAME AS INDEXNAME, 
               B.INDEX_ID, 
               D.NAME AS COLUMNNAME, 
               C.KEY_ORDINAL 
        FROM   SYS.OBJECTS A 
               INNER JOIN SYS.INDEXES B 
                 ON A.OBJECT_ID = B.OBJECT_ID 
               INNER JOIN SYS.INDEX_COLUMNS C 
                 ON B.OBJECT_ID = C.OBJECT_ID 
                    AND B.INDEX_ID = C.INDEX_ID 
               INNER JOIN SYS.COLUMNS D 
                 ON C.OBJECT_ID = D.OBJECT_ID 
                    AND C.COLUMN_ID = D.COLUMN_ID 
        WHERE  A.TYPE <> 'S') P 
       PIVOT 
       (MIN(COLUMNNAME) 
        FOR KEY_ORDINAL IN ( [1],[2],[3],[4],[5],[6],[7] ) ) AS PVT 
       INNER JOIN SYS.DM_DB_INDEX_USAGE_STATS B 
         ON PVT.OBJECT_ID = B.OBJECT_ID 
            AND PVT.INDEX_ID = B.INDEX_ID 
            AND B.DATABASE_ID = DB_ID() 
UNION  
SELECT TABLENAME, INDEXNAME, INDEX_ID, [1] AS COL1, [2] AS COL2, [3] AS COL3, 
       [4] AS COL4, [5] AS COL5, [6] AS COL6, [7] AS COL7, 0, 0, 0 
FROM   (SELECT A.NAME AS TABLENAME, 
               A.OBJECT_ID, 
               B.NAME AS INDEXNAME, 
               B.INDEX_ID, 
               D.NAME AS COLUMNNAME, 
               C.KEY_ORDINAL 
        FROM   SYS.OBJECTS A 
               INNER JOIN SYS.INDEXES B 
                 ON A.OBJECT_ID = B.OBJECT_ID 
               INNER JOIN SYS.INDEX_COLUMNS C 
                 ON B.OBJECT_ID = C.OBJECT_ID 
                    AND B.INDEX_ID = C.INDEX_ID 
               INNER JOIN SYS.COLUMNS D 
                 ON C.OBJECT_ID = D.OBJECT_ID 
                    AND C.COLUMN_ID = D.COLUMN_ID 
        WHERE  A.TYPE <> 'S') P 
       PIVOT 
       (MIN(COLUMNNAME) 
        FOR KEY_ORDINAL IN ( [1],[2],[3],[4],[5],[6],[7] ) ) AS PVT 
WHERE  NOT EXISTS (SELECT OBJECT_ID, 
                          INDEX_ID 
                   FROM   SYS.DM_DB_INDEX_USAGE_STATS B 
                   WHERE  DATABASE_ID = DB_ID(DB_NAME()) 
                          AND PVT.OBJECT_ID = B.OBJECT_ID 
                          AND PVT.INDEX_ID = B.INDEX_ID) 
ORDER BY TABLENAME, INDEX_ID;

Next Steps

The above queries should give you a jump start to determine how indexes are being used and which indexes are being used. This will allow you to remove unused indexes as well as look for duplicate indexes that can be removed.
Based on the information collected you can determine which indexes can safely be dropped. Just make sure you collect stats long enough to have a good sampling of queries that are run against your database.
Now that you know how to determine which unused indexes you can get rid of, stay tuned for future tips on how to determine which indexes are needed

Last Update: 7/22/2008

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Comments and Feedback:

Tuesday, July 22, 2008 - 2:12:34 AM - Jensch

Read The Tip

Realy nice statements,but my SSMS seems to dislike Query 3.
It disconnects at every attempt executing the query.
Any idea as to this?

Best regards
- Jens

Tuesday, July 22, 2008 - 4:50:01 AM - admin

Read The Tip

Not sure why this would be the case.

The one issue I have found is if you have a case sensitive database the upper case causes an issue.

Wednesday, July 23, 2008 - 1:44:43 AM - ESL

Read The Tip

Hi,

sorry but all the request with PIVOT option doesn't work, error is :

Line 20: Incorrect syntax near 'PIVOT'.

do you have an issue to correct .

regards ,

Eric

Wednesday, July 23, 2008 - 5:22:29 AM - grobido	Read The Tip
Are you using SQL 2005? PIVOT only works with SQL Server 2005 and up.

Wednesday, July 23, 2008 - 5:33:12 AM - ESL

Read The Tip

Yes i do,

i have the solution, this is beacause QUOTED_IDENTIFIER is not set on

thank's

Tuesday, August 05, 2008 - 1:59:19 AM - Rimsky

Read The Tip

Very valuable, thank you. I have a number of databases with the same structure and indexes. Is there a way I can run the following query on all databases? I think I need to join to master..sysdatabases where databasename like 'McBam%' , but how do I do that?

SELECT DB_NAME() AS DATABASENAME,

OBJECT_NAME(B.OBJECT_ID) AS TABLENAME,

B.NAME AS INDEXNAME,

B.INDEX_ID

FROM SYS.OBJECTS A

INNER JOIN SYS.INDEXES B

ON A.OBJECT_ID = B.OBJECT_ID

WHERE NOT EXISTS (SELECT *

FROM SYS.DM_DB_INDEX_USAGE_STATS C

WHERE B.OBJECT_ID = C.OBJECT_ID

AND B.INDEX_ID = C.INDEX_ID)

AND A.TYPE <> 'S'

AND B.NAME > ''

AND B.NAME not like '%rowid%'

ORDER BY 1, 2, 3

Tuesday, August 05, 2008 - 7:11:07 AM - aprato

Read The Tip

Use sp_MSForEachDB. This should not be used as production code.

Double check query too... I didn't test it.

sp_MSForEachDB 'use ?;
                declare @name sysname
                set @name = db_name()
                if @name like ''McBam%''
                   SELECT DB_NAME() AS DATABASENAME, OBJECT_NAME(B.OBJECT_ID) AS TABLENAME, B.NAME AS INDEXNAME, B.INDEX_ID
                   FROM SYS.OBJECTS A
                   INNER JOIN SYS.INDEXES B ON A.OBJECT_ID = B.OBJECT_ID
                   WHERE NOT EXISTS (SELECT 1
                                     FROM SYS.DM_DB_INDEX_USAGE_STATS C
                                     WHERE B.OBJECT_ID = C.OBJECT_ID
                                     AND B.INDEX_ID = C.INDEX_ID)
                                     AND A.TYPE <> ''S''
                                     AND B.NAME > ''''
                                     AND B.NAME not like ''%rowid%''
                                     ORDER BY 1, 2, 3'

Thursday, September 02, 2010 - 8:34:45 AM - Douglas Osborne

Read The Tip

I tweaked query 7 a bit since we have a few really long indexes - so a quick way to find your limit is to keep bumping up the pivot number until you return no rows, I had to go to [10] - eg

SELECT *
FROM
(
    SELECT A.NAME AS TABLENAME, A.OBJECT_ID, B.NAME AS INDEXNAME, B.INDEX_ID, D.NAME AS COLUMNNAME, C.KEY_ORDINAL
    FROM SYS.OBJECTS A
    INNER JOIN SYS.INDEXES B ON A.OBJECT_ID = B.OBJECT_ID
    INNER JOIN SYS.INDEX_COLUMNS C ON B.OBJECT_ID = C.OBJECT_ID AND B.INDEX_ID = C.INDEX_ID
    INNER JOIN SYS.COLUMNS D ON C.OBJECT_ID = D.OBJECT_ID AND C.COLUMN_ID = D.COLUMN_ID
    WHERE A.TYPE <> 'S'
) P
PIVOT
(MIN(COLUMNNAME)
FOR KEY_ORDINAL IN ( [1],[2],[3],[4],[5],[6],[7], [8], [9], [10] ) ) AS PVT
INNER JOIN SYS.DM_DB_INDEX_USAGE_STATS B ON PVT.OBJECT_ID = B.OBJECT_ID AND PVT.INDEX_ID = B.INDEX_ID AND B.DATABASE_ID = DB_ID()
WHERE [10] IS NOT NULL

Nice script BTW,

Doug

Thursday, June 02, 2011 - 11:42:19 PM - Ludwig

Read The Tip

Great article...that is surely helpful.

I have also heard that even though it seems an index is not used (counters 0 all over) system performance goes down once an index classified as unused is removed...therefore somehow in the back the index seems to be used even though it states it is not being used...not sure but maybe the index is used in a way that does not trigger the tracking of its usage.. Anyone with simillar expierence?

Thx Ludwig

Tuesday, August 07, 2012 - 3:32:21 PM - Brandy	Read The Tip
Has anyone figured out how to add the "included" fields into these queries for SQL 2008 and above?

Thursday, November 15, 2012 - 8:54:28 AM - RGSKAL

Read The Tip

First, I am grateful of Greg forposting this great article. I have implemented and am storing all the information into a table for continous usage info, but I like to understand how to compensate for a bounce of the server or attach/detach as DMV is going to be reset? My heap is going to have all the information before a bounce takes place, but how do I use this information continously after the bounce? Appreciate an answer. Regards!

Thursday, November 15, 2012 - 9:09:06 AM - Greg Robidoux

Read The Tip

@RGSKAL - take a look at these tips:

http://www.mssqltips.com/sqlservertip/1749/retaining-historical-index-usage-statistics-for-sql-server-part-1-of-3/

http://www.mssqltips.com/sqlservertip/1766/retaining-historical-index-usage-statistics-for-sql-server-part-2-of-3/

http://www.mssqltips.com/sqlservertip/1789/retaining-historical-index-usage-statistics-for-sql-server-part-3-of-3/

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