Index Statistics and AUTO_UPDATE

I recently worked with a client that uses a SQL Server 2008 data warehouse and was having performance issues while loading data. There were a number of issues to address, but one of the issues surrounded table statistics – more specifically, how and when they were to be maintained.

AUTO_UPDATE_STATISTICS

The first issue was the question about the load performance impact of auto stats. My client’s concern was that if this option was enabled that SQL Server would waste resources computing statistics over and over again while data was being loaded into the tables. The good news is that SQL Server does not compute statistics as data is loading into a table. The auto stats does not compute statistics until they are needed.

Take for example the following:

use master

–Disable row counting

SET NOCOUNT ON

–Drop the DemoDatabase if it exists

IF EXISTS (SELECT * FROM sys.databases WHERE name = ‘DemoDatabase’)

BEGIN

ALTER DATABASE DemoDatabase SET SINGLE_USER WITH ROLLBACK IMMEDIATE

DROP DATABASE DemoDatabase

END

–Create the DemoDatabase

CREATE DATABASE DemoDatabase

–Make sure auto-stats is enabled

ALTER DATABASE DemoDatabase SET AUTO_UPDATE_STATISTICS ON

–Change database context

use DemoDatabase

–Create a test table

CREATE TABLE dbo.TestTable

(

ClusterColumn INT NOT NULL IDENTITY(1, 1),

NonClusterColumn INT NOT NULL

)

–Create a clustered index on the test table

CREATE UNIQUE CLUSTERED INDEX PK_TestTable ON dbo.TestTable

(

ClusterColumn

)

–Create a non-clustered index on the test table

CREATE NONCLUSTERED INDEX IX_NonClusterColumn ON dbo.TestTable

(

NonClusterColumn

)

/*Check the last update date for the statistics on each table

NOTE: This will be NULL as the statistics will not *exist* yet*/

SELECT

STATS_DATE(OBJECT_ID(‘dbo.TestTable’), INDEXPROPERTY(OBJECT_ID(‘dbo.TestTable’), ‘PK_TestTable’, ‘IndexID’)) AS ClusterLastUpdate,

STATS_DATE(OBJECT_ID(‘dbo.TestTable’), INDEXPROPERTY(OBJECT_ID(‘dbo.TestTable’), ‘IX_NonClusterColumn’, ‘IndexID’)) AS NonClusterLastUpdate

–Load some data into the table

DECLARE @I INT = 0

WHILE @I < 1000

BEGIN

INSERT INTO dbo.TestTable (NonClusterColumn) VALUES(@I)

SET @I += 1

END

/*Check the last update date for the statistics on each table

NOTE: This will be still be NULL as the statistics have not been

required yet and as such have not been created by auto stats*/

SELECT

STATS_DATE(OBJECT_ID(‘dbo.TestTable’), INDEXPROPERTY(OBJECT_ID(‘dbo.TestTable’), ‘PK_TestTable’, ‘IndexID’)) AS ClusterLastUpdate,

STATS_DATE(OBJECT_ID(‘dbo.TestTable’), INDEXPROPERTY(OBJECT_ID(‘dbo.TestTable’), ‘IX_NonClusterColumn’, ‘IndexID’)) AS NonClusterLastUpdate

–Execute a query that needs statistics

SELECT

COUNT(*)

FROM

dbo.TestTable

WHERE

NonClusterColumn BETWEEN 100 AND 200

/*Check the last update date for the statistics on each table

NOTE: statistics exist now for the non-clustered index because

they were needed to execute the previous select query but still

don’t exist for the clustered index as it was not needed to execute this query*/

SELECT

STATS_DATE(OBJECT_ID(‘dbo.TestTable’), INDEXPROPERTY(OBJECT_ID(‘dbo.TestTable’), ‘PK_TestTable’, ‘IndexID’)) AS ClusterLastUpdate,

STATS_DATE(OBJECT_ID(‘dbo.TestTable’), INDEXPROPERTY(OBJECT_ID(‘dbo.TestTable’), ‘IX_NonClusterColumn’, ‘IndexID’)) AS NonClusterLastUpdate

Rebuilding Indexes

Another point that was brought up during these discussions was whether it is necessary to update statistics for an index after the index is rebuilt. The answer is no. Rebuilding an index will automatically update your statistics even if AUTO_UPDATE_STATISTICS is set to off. This does not hold true however for reorganizing (defragmenting) an index. Only index rebuilds.

Filed under: Programming SQL Server, SQL Server 2008, SQL Server Deployment, SQL Server Programming, T-SQL, Uncategorized Tagged: AUTO_UPDATE_STATISTICS, Index Performance, Indexes, Optimization, Query Performance, Statistics Image may be NSFW.
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Image may be NSFW.
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Index Statistics and AUTO_UPDATE_STATISTICS

AUTO_UPDATE_STATISTICS

Rebuilding Indexes

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