Saturday, May 14, 2011

Get Read-to-Write Ratio for All Indexes

Here's a quick script to help you quantify how useful your indexes are, in the sense of how much they're used to speed up queries, compared to how much work it is for SQL Server to maintain them during inserts, updates, and deletes. This only reflects the usage since the last SQL Server boot.

-- Displays the read-to-update ratio of all indexes.  Good for finding ones that may not be needed.
DECLARE @dbid INT = DB_ID(DB_NAME())

; WITH cteUser AS
(
    SELECT object_id,
           index_id,
           user_seeks + user_scans + user_lookups   AS 'User Reads',
           user_updates                             AS 'User Updates'
      FROM sys.dm_db_index_usage_stats   
     WHERE database_id = @dbid
)
,
cteSystem AS
(
    SELECT object_id,
           index_id,
           system_seeks + system_scans + system_lookups   AS 'System Reads',
           system_updates                                 AS 'System Updates'
      FROM sys.dm_db_index_usage_stats   
     WHERE database_id = @dbid
)
,
cteTotal AS
(
    SELECT u.object_id,
           u.index_id,
           [User Reads]   + [System Reads]     AS 'Total Reads',
           [User Updates] + [System Updates]   AS 'Total Updates'
      FROM cteUser     u
      JOIN cteSystem   s
        ON u.object_id = s.object_id
       AND u.index_id  = s.index_id
)
,
cteReadToUpdateRatio AS
(
    SELECT object_id,
           index_id,
           CONVERT(NVARCHAR,
                  CONVERT(MONEY, ISNULL(
                                        CAST([Total Reads] AS REAL)
                                        /
                                        NULLIF([Total Updates], 0.0)
                                        , [Total Reads]
                                        )
                               , 1
                         )
                  )   AS 'Read-to-Update Ratio'
                  
      FROM cteTotal
)
SELECT OBJECT_SCHEMA_NAME(i.object_id)                                                                  AS 'Schema Name',
       OBJECT_NAME(i.object_id)                                                                         AS 'Table Name',
       i.name                                                                                           AS 'Index Name',
       REPLACE(CONVERT(NVARCHAR, CONVERT(MONEY, u.[User Reads]),                      1), '.00', '')    AS 'User Reads',                   
       REPLACE(CONVERT(NVARCHAR, CONVERT(MONEY, u.[User Updates]),                    1), '.00', '')    AS 'User Updates',                 
       REPLACE(CONVERT(NVARCHAR, CONVERT(MONEY, s.[System Reads]),                    1), '.00', '')    AS 'System Reads',                 
       REPLACE(CONVERT(NVARCHAR, CONVERT(MONEY, s.[System Updates]),                  1), '.00', '')    AS 'System Updates',               
       REPLACE(CONVERT(NVARCHAR, CONVERT(MONEY, t.[Total Reads]),                     1), '.00', '')    AS 'Total Reads',                
       REPLACE(CONVERT(NVARCHAR, CONVERT(MONEY, t.[Total Updates]),                   1), '.00', '')    AS 'Total Updates',                
                                                r.[Read-to-Update Ratio]                                AS 'Read-to-Update Ratio',         
       REPLACE(CONVERT(NVARCHAR, CONVERT(MONEY, t.[Total Reads] + t.[Total Updates]), 1), '.00', '')    AS 'Total Reads + Total Updates'
  FROM cteUser                        u
  JOIN cteSystem                      s
    ON u.object_id = s.object_id
   AND u.index_id  = s.index_id
  JOIN cteTotal                       t
    ON u.object_id = t.object_id
   AND u.index_id  = t.index_id
  JOIN cteReadToUpdateRatio           r
    ON u.object_id = r.object_id
   AND u.index_id  = r.index_id
  JOIN sys.indexes                    i
    ON u.object_id = i.object_id
   AND u.index_id  = i.index_id
  JOIN sys.objects                    o
    ON u.object_id = o.object_id
 WHERE OBJECTPROPERTY(o.object_id, 'IsUserTable') = 1
   AND i.is_primary_key = 0
 ORDER BY CAST(r.[Read-to-Update Ratio] AS REAL)

The results appear in "least useful" index first. Be sure to read and test this script before using in a Production environment.

4 comments:

Tom said...

It would help if the first column of the final select showed the schema name.
OBJECT_SCHEMA_NAME (i.object_id) AS 'Schema Name',

Anonymous said...

This is misleading, as it does not quantify the importance of indices in relation to business processes. The qualitative issues overwhelm any quantitative.

And if you had a smart optimizer, it might decide to use different indices based on data volume and distribution.

So explain it to the controller when the end-of-year processing crawls because no one remembered that's what the obscure index was for.

Cedric said...

Hi Leonard

The code is missing a "," between teh columns

'Read-to-Update Ratio' and 'Total Reads + Total Updates'

Just a heads up ,thanks for the query ,would recommend stating the Ratio's as percentages as well, I have altered the code to do exactly that but the comments box is a little small to post the changes I made .

Larry Leonard said...

Anonymous - Obviously, you'd have to use a little common sense. This is just a starting point for investigations. And of course indexes that only get used once a year might be good candidates for documentation, perhaps even in the extended property of the index itself, and maybe even enforced via permissions or DDL trigger on DROPing the index. The possibilities are endless, if you think about it; but this article was hardly meant to be a full description of index management!

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