Knowing when and how often your indexes are used can really come in handy. Indexes that are never utilized aren’t worth keeping around, and knowing their usage patterns can be a big help when making decisions regarding things such as filegroup placement and compression settings. SQL Server 2005 brought some great advancements in the form of the sys.dm_db_index_usage_stats DMV, which returns statistics on index usage that are automatically being kept by the server. While it’s a step forward, I feel it still leaves a few things to be desired:
- The values are cumulative since the server was last restarted
- Once the server is shut down or restarted, there’s no way to get the previous values back
With this in mind, I came up with my own system for archiving non-cumulative index usage stats on a daily basis. Microsoft introduced Management Data Warehouse in SQL Server 2008 which provides a way to keep track of these statistics over time (along with many others), but I’ve been using my own method longer than that. This will also work in SQL Server Express Edition, which doesn’t support MDW. I’ll give descriptions of each part below, but if you just want the code right now download it here.
Tables
IndexUsageStats_LastCumulative
As I mentioned before, the output from sys.dm_db_index_usage_stats is cumulative since the last time the server was restarted, and I was looking for a way to find daily values that weren’t cumulative. Restarting the server immediately after collecting the values would have accomplished this, but users probably wouldn’t be very happy :). Instead I just keep track of the previous cumulative values and subtract them from the current ones. If the output shows an index was used 7 times on Tuesday at Midnight, and the same number was 12 on Wednesday at Midnight, then it must have been used 5 times in that 24 hour period. IndexUsageStats_LastCumulative holds the previous cumulative values for all indexes to make this calculation possible.
Names
The only true way to identify an object in SQL Server is by its name, but DMVs typically return object IDs instead of names. It’s easy to translate between names and IDs, but over time names and IDs can change:
- If an object is dropped and re-created with the same name, chances of it having the same ID are virtually zero.
- If an object is renamed, its ID will not change, but of course its name will.
Using either IDs or Names exclusively can make historical values become useless over time – you might have an ID that’s no longer valid or a name that changed a while back. To give myself more options for tracking changes over time, I store both. Since storing each object’s name in a character data type would consume a lot of space and be repetitive, I prefer to map object names to numeric values, and this table facilitates that.
IndexUsageStats
This table does the heavy lifting storing for our system – it contains all the historical index usage statistics by both object ID and name. I’ve found it to be a great candidate for compression if you’re running enterprise edition, but it’s still pretty compact even when uncompressed thanks to the Names table.
Stored Procedures
CollectIndexUsageStats
The stored procedure that drives statistics collection. You’ll want to set up a job to run this – mine runs at midnight. Basically it iterates through each online database in the instance collecting usage stats and then compares them to the previous cumulative values and writes the differences to the IndexUsageStats table. You’ll find comments in-line.
Views
vw_IndexUsageStats
Since the data contained in the IndexUsageStats table is all numeric, it’s not the easiest to browse. This view makes some joins back to the Names table so things are a little more user-friendly. Most of the queries I write are against this view. Object IDs aren’t returned here as they’re typically not needed. When they are necessary, I’ll write more detailed queries against IndexUsageStats.
A few notes before the code:
- As always, this code is offered as-is with no warranty whatsoever. Do not deploy this in a production environment before you have tested it and understand exactly what it does.
- I recommend putting all these objects in their own database. If you already have a DBA database for management information that should work fine too. My database is called “iDBA” because I inherited it with that name.
- This was written and tested on SQL Server 2008 and 2008R2. It can probably be tweaked to run in 2005 by removing the datetime2 datatype and writing substitute queries for the MERGE statement.
----------------------------------------------------------- -- Name: CollectIndexUsageStats.sql -- -- Description: Collects index usage statistics over time. -- -- Author: Bob Pusateri, http://www.bobpusateri.com -- -- THIS SCRIPT IS PROVIDED "AS-IS" WITHOUT ANY WARRANTY. -- DO NOT RUN THIS ON A PRODUCTION SYSTEM UNTIL YOU HAVE -- COMPLETE UNDERSTANDING OF THE TASKS IT PERFORMS AND -- HAVE TESTED IT ON A DEVELOPMENT SYSTEM. ----------------------------------------------------------- -- I recommend putting these objects in a separate database -- (or your existing DBA database if you have one) -- -- If your DBA database is not named 'iDBA', you'll want to replace it with your DB name -- It is referenced several times in the script below USE [iDBA]; GO -- stores cumulative data from sys.dm_db_index_usage_stats DMV CREATE TABLE [dbo].[IndexUsageStats_LastCumulative] ( [ServerNameID] [int] NOT NULL, [DatabaseID] [smallint] NOT NULL, [ObjectID] [int] NOT NULL, [IndexID] [int] NOT NULL, [LoadTime] [datetime2](0) NOT NULL, [User_Seeks] [bigint] NOT NULL, [User_Scans] [bigint] NOT NULL, [User_Lookups] [bigint] NOT NULL, [User_Updates] [bigint] NOT NULL, [System_Seeks] [bigint] NOT NULL, [System_Scans] [bigint] NOT NULL, [System_Lookups] [bigint] NOT NULL, [System_Updates] [bigint] NOT NULL, CONSTRAINT [PK_IUS_C] PRIMARY KEY CLUSTERED ([ServerNameID],[DatabaseID],[ObjectID],[IndexID]) ); GO -- used for Server/DB/Schema/Table/Index name mapping CREATE TABLE [dbo].[Names] ( [ID] [int] IDENTITY(1,1) NOT NULL, [Value] [nvarchar](260) NOT NULL, CONSTRAINT [PK_Names] PRIMARY KEY CLUSTERED ([ID]) ); GO -- stores historical usage statistics CREATE TABLE [dbo].[IndexUsageStats] ( [StatsDate] [datetime2](0) NOT NULL, [ServerNameID] [int] NOT NULL, [DatabaseID] [smallint] NOT NULL, [ObjectID] [int] NOT NULL, [IndexID] [int] NOT NULL, [DatabaseNameID] [int] NOT NULL, [SchemaNameID] [int] NOT NULL, [TableNameID] [int] NOT NULL, [IndexNameID] [int] NULL, [User_Seeks] [bigint] NOT NULL, [User_Scans] [bigint] NOT NULL, [User_Lookups] [bigint] NOT NULL, [User_Updates] [bigint] NOT NULL, [System_Seeks] [bigint] NOT NULL, [System_Scans] [bigint] NOT NULL, [System_Lookups] [bigint] NOT NULL, [System_Updates] [bigint] NOT NULL, CONSTRAINT [PK_IUS] PRIMARY KEY CLUSTERED ([StatsDate],[ServerNameID],[DatabaseID],[ObjectID],[IndexID]), CONSTRAINT [FK_IUS_Names_DB] FOREIGN KEY([DatabaseNameID]) REFERENCES [dbo].[Names] ([ID]), CONSTRAINT [FK_IUS_Names_Index] FOREIGN KEY([IndexNameID]) REFERENCES [dbo].[Names] ([ID]), CONSTRAINT [FK_IUS_Names_Schema] FOREIGN KEY([SchemaNameID]) REFERENCES [dbo].[Names] ([ID]), CONSTRAINT [FK_IUS_Names_Table] FOREIGN KEY([TableNameID]) REFERENCES [dbo].[Names] ([ID]), CONSTRAINT [CK_IUS_PositiveValues] CHECK ([User_Seeks]>=(0) AND [User_Scans]>=(0) AND [user_Lookups]>=(0) AND [user_updates]>=(0) AND [system_seeks]>=(0) AND [system_scans]>=(0) AND [system_lookups]>=(0) AND [system_updates]>=(0)) ); GO -- collects usage statistics -- I run this once daily (can be run more often if you like) CREATE PROCEDURE [dbo].[CollectIndexUsageStats] AS BEGIN BEGIN TRY SET NOCOUNT ON; -- get current stats for all online databases SELECT database_id, name INTO #dblist FROM sys.databases WHERE [state] = 0 AND database_id != 2; -- skip TempDB CREATE TABLE #t ( StatsDate DATETIME2(0), ServerName SYSNAME, DatabaseID SMALLINT, ObjectID INT, IndexID INT, DatabaseName SYSNAME, SchemaName SYSNAME, TableName SYSNAME, IndexName SYSNAME NULL, User_Seeks BIGINT, User_Scans BIGINT, User_Lookups BIGINT, User_Updates BIGINT, System_Seeks BIGINT, System_Scans BIGINT, System_Lookups BIGINT, System_Updates BIGINT ); DECLARE @DBID INT; DECLARE @DBNAME SYSNAME; DECLARE @Qry NVARCHAR(2000); -- iterate through each DB, generate & run query WHILE (SELECT COUNT(*) FROM #dblist) > 0 BEGIN SELECT TOP (1) @DBID=database_id, @DBNAME=[name] FROM #dblist ORDER BY database_id; SET @Qry = ' INSERT INTO #t SELECT SYSDATETIME() AS StatsDate, @@SERVERNAME AS ServerName, s.database_id AS DatabaseID, s.object_id AS ObjectID, s.index_id AS IndexID, ''' + @DBNAME + ''' AS DatabaseName, c.name AS SchemaName, o.name AS TableName, i.name AS IndexName, s.user_seeks, s.user_scans, s.user_lookups, s.user_updates, s.system_seeks, s.system_scans, s.system_lookups, s.system_updates FROM sys.dm_db_index_usage_stats s INNER JOIN ' + @DBNAME + '.sys.objects o ON s.object_id = o.object_id INNER JOIN ' + @DBNAME + '.sys.schemas c ON o.schema_id = c.schema_id INNER JOIN ' + @DBNAME + '.sys.indexes i ON s.object_id = i.object_id and s.index_id = i.index_id WHERE s.database_id = ' + CONVERT(NVARCHAR,@DBID) + '; '; EXEC sp_executesql @Qry; DELETE FROM #dblist WHERE database_id = @DBID; END -- db while loop DROP TABLE #DBList; BEGIN TRAN; -- create ids for Server Name by inserting new ones into dbo.Names INSERT INTO iDBA.dbo.Names (Value) SELECT DISTINCT RTRIM(LTRIM(t.ServerName)) AS ServerName FROM #t t LEFT JOIN iDBA.dbo.Names n ON t.ServerName = n.Value WHERE n.ID IS NULL AND t.ServerName IS NOT NULL ORDER BY RTRIM(LTRIM(t.ServerName)); -- same as above for DatabaseName INSERT INTO iDBA.dbo.Names (Value) SELECT DISTINCT RTRIM(LTRIM(t.DatabaseName)) AS DatabaseName FROM #t t LEFT JOIN iDBA.dbo.Names n ON t.DatabaseName = n.Value WHERE n.ID IS NULL AND t.DatabaseName IS NOT NULL ORDER BY RTRIM(LTRIM(t.DatabaseName)); -- SchemaName INSERT INTO iDBA.dbo.Names (Value) SELECT DISTINCT RTRIM(LTRIM(t.SchemaName)) AS SchemaName FROM #t t LEFT JOIN iDBA.dbo.Names n ON t.SchemaName = n.Value WHERE n.ID IS NULL AND t.SchemaName IS NOT NULL ORDER BY RTRIM(LTRIM(t.SchemaName)); -- TableName INSERT INTO iDBA.dbo.Names (Value) SELECT DISTINCT RTRIM(LTRIM(t.TableName)) AS TableName FROM #t t LEFT JOIN iDBA.dbo.Names n ON t.TableName = n.Value WHERE n.ID IS NULL AND t.TableName IS NOT NULL ORDER BY RTRIM(LTRIM(t.TableName)); -- IndexName INSERT INTO iDBA.dbo.Names (Value) SELECT DISTINCT RTRIM(LTRIM(t.IndexName)) AS IndexName FROM #t t LEFT JOIN iDBA.dbo.Names n ON t.IndexName = n.Value WHERE n.ID IS NULL AND t.IndexName IS NOT NULL ORDER BY RTRIM(LTRIM(t.IndexName)); -- Calculate Deltas INSERT INTO iDBA.dbo.IndexUsageStats (StatsDate, ServerNameID, DatabaseID, ObjectID, IndexID, DatabaseNameID, SchemaNameID, TableNameID, IndexNameID, User_Seeks, User_Scans, User_Lookups, User_Updates, System_Seeks, System_Scans, System_Lookups, System_Updates) SELECT t.StatsDate, s.ID AS ServerNameID, t.DatabaseID, t.ObjectID, t.IndexID, d.ID AS DatabaseNameID, c.ID AS SchemaNameID, b.ID AS TableNameID, i.ID AS IndexNameID, CASE -- if the previous cumulative value is greater than the current one, the server has been reset -- just use the current value WHEN t.User_Seeks - ISNULL(lc.User_Seeks,0) < 0 THEN t.User_Seeks -- if the prev value is less than the current one, then subtract to get the delta ELSE t.User_Seeks - ISNULL(lc.User_Seeks,0) END AS User_Seeks, CASE WHEN t.User_Scans - ISNULL(lc.User_Scans,0) < 0 THEN t.User_Scans ELSE t.User_Scans - ISNULL(lc.User_Scans,0) END AS User_Scans, CASE WHEN t.User_Lookups - ISNULL(lc.User_Lookups,0) < 0 THEN t.User_Lookups ELSE t.User_Lookups - ISNULL(lc.User_Lookups,0) END AS User_Lookups, CASE WHEN t.User_Updates - ISNULL(lc.User_Updates,0) < 0 THEN t.User_Updates ELSE t.User_Updates - ISNULL(lc.User_Updates,0) END AS User_Updates, CASE WHEN t.System_Seeks - ISNULL(lc.System_Seeks,0) < 0 THEN t.System_Seeks ELSE t.System_Seeks - ISNULL(lc.System_Seeks,0) END AS System_Seeks, CASE WHEN t.System_Scans - ISNULL(lc.System_Scans,0) < 0 THEN t.System_Scans ELSE t.System_Scans - ISNULL(lc.System_Scans,0) END AS System_Scans, CASE WHEN t.System_Lookups - ISNULL(lc.System_Lookups,0) < 0 THEN t.System_Lookups ELSE t.System_Lookups - ISNULL(lc.System_Lookups,0) END AS System_Lookups, CASE WHEN t.System_Updates - ISNULL(lc.System_Updates,0) < 0 THEN t.System_Updates ELSE t.System_Updates - ISNULL(lc.System_Updates,0) END AS System_Updates FROM #t t INNER JOIN iDBA.dbo.Names s ON t.ServerName = s.Value INNER JOIN iDBA.dbo.Names d ON t.DatabaseName = d.Value INNER JOIN iDBA.dbo.Names c ON t.SchemaName = c.Value INNER JOIN iDBA.dbo.Names b ON t.TableName = b.Value LEFT JOIN iDBA.dbo.Names i ON t.IndexName = i.Value LEFT JOIN iDBA.dbo.IndexUsageStats_LastCumulative lc ON s.ID = lc.ServerNameID AND t.DatabaseID = lc.DatabaseID AND t.ObjectID = lc.ObjectID AND t.IndexID = lc.IndexID ORDER BY StatsDate, ServerName, DatabaseID, ObjectID, IndexID; -- Update last cumulative values with the current ones MERGE INTO iDBA.dbo.IndexUsageStats_LastCumulative lc USING #t t INNER JOIN iDBA.dbo.Names s ON t.ServerName = s.Value ON s.ID = lc.ServerNameID AND t.DatabaseID = lc.DatabaseID AND t.ObjectID = lc.ObjectID AND t.IndexID = lc.IndexID WHEN MATCHED THEN UPDATE SET lc.LoadTime = t.StatsDate, lc.User_Seeks = t.User_Seeks, lc.User_Scans = t.User_Scans, lc.User_Lookups = t.User_Lookups, lc.User_Updates = t.User_Updates, lc.System_Seeks = t.System_Seeks, lc.System_Scans = t.System_Scans, lc.System_Lookups = t.System_Lookups, lc.System_Updates = t.System_Updates WHEN NOT MATCHED BY TARGET THEN INSERT (ServerNameID, DatabaseID, ObjectID, IndexID, LoadTime, User_Seeks, User_Scans, User_Lookups, User_Updates, System_Seeks, System_Scans, System_Lookups, System_Updates) VALUES (s.ID, t.DatabaseID, t.ObjectID, t.IndexID, t.StatsDate, t.User_Seeks, t.User_Scans, t.User_Lookups, t.User_Updates, t.System_Seeks, t.System_Scans, t.System_Lookups, t.System_Updates) WHEN NOT MATCHED BY SOURCE THEN DELETE; COMMIT TRAN; END TRY BEGIN CATCH IF @@TRANCOUNT > 0 ROLLBACK TRAN; DECLARE @ErrorNumber INT; DECLARE @ErrorSeverity INT; DECLARE @ErrorState INT; DECLARE @ErrorProcedure NVARCHAR(126); DECLARE @ErrorLine INT; DECLARE @ErrorMessage NVARCHAR(2048); SELECT @ErrorNumber = ERROR_NUMBER(), @ErrorSeverity = ERROR_SEVERITY(), @ErrorState = ERROR_STATE(), @ErrorProcedure = ERROR_PROCEDURE(), @ErrorLine = ERROR_LINE(), @ErrorMessage = ERROR_MESSAGE(); RAISERROR (@ErrorMessage, @ErrorSeverity, @ErrorState); END CATCH END GO -- displays usage statistics CREATE VIEW [dbo].[vw_IndexUsageStats] AS SELECT s.StatsDate, vn.Value AS ServerName, dbn.Value AS DatabaseName, sn.Value AS SchemaName, tn.Value AS TableName, dn.Value AS IndexName, s.IndexID, s.User_Seeks, s.User_Scans, s.User_Lookups, s.User_Updates, s.System_Seeks, s.System_Scans, s.System_Lookups, s.System_Updates FROM dbo.IndexUsageStats s INNER JOIN dbo.Names vn ON s.ServerNameID = vn.ID INNER JOIN dbo.Names dbn ON s.DatabaseNameID = dbn.ID INNER JOIN dbo.Names sn ON s.SchemaNameID = sn.ID INNER JOIN dbo.Names tn ON s.TableNameID = tn.ID LEFT JOIN dbo.Names dn ON s.IndexNameID = dn.ID; GO