How to Update Statistics
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Posted on June 20, 2009 by Derek Dieter
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1
There are two ways to update statistics. The first way is the easy way. It is one line of code that will update all the statistics in the database using the default sample size of 20,000 rows per table.
The other way, is to use the UPDATE STATISTICS command. This command gives much better granularity of control:
UPDATE STATISTICS Sales.SalesOrderDetail
-- Update a specific index on a table
UPDATE STATISTICS Sales.SalesOrderDetail IX_SalesOrderDetail
-- Update one column on a table specifying sample size
UPDATE STATISTICS Production.Product(Products) WITH SAMPLE 50 PERCENT
Using update statistics can give you the granularity of control to only update the out of date statistics, thus having less impact on your production system.
The following script updates all out of date statistics. Set the @MaxDaysOld variable to the number of days you will allow the statistics to be out of date by. Setting the @SamplePercent variable to null will use the SQL Server default value of 20,000 rows. You can also change the sample type to specify rows or percent.
DECLARE @SamplePercent INT
DECLARE @SampleType nvarchar(50)
SET @MaxDaysOld = 0
SET @SamplePercent = NULL --25
SET @SampleType = 'PERCENT' --'ROWS'
BEGIN TRY
DROP TABLE #OldStats
END TRY
BEGIN CATCH SELECT 1 END CATCH
SELECT
RowNum = ROW_NUMBER() OVER (ORDER BY ISNULL(STATS_DATE(object_id, st.stats_id),1))
,TableName = OBJECT_SCHEMA_NAME(st.object_id) + '.' + OBJECT_NAME(st.object_id)
,StatName = st.name
,StatDate = ISNULL(STATS_DATE(object_id, st.stats_id),1)
INTO #OldStats
FROM sys.stats st WITH (nolock)
WHERE DATEDIFF(DAY, ISNULL(STATS_DATE(object_id, st.stats_id),1), GETDATE()) > @MaxDaysOld
ORDER BY ROW_NUMBER() OVER (ORDER BY ISNULL(STATS_DATE(object_id, st.stats_id),1))
DECLARE @MaxRecord INT
DECLARE @CurrentRecord INT
DECLARE @TableName nvarchar(255)
DECLARE @StatName nvarchar(255)
DECLARE @SQL nvarchar(MAX)
DECLARE @SampleSize nvarchar(100)
SET @MaxRecord = (SELECT MAX(RowNum) FROM #OldStats)
SET @CurrentRecord = 1
SET @SQL = ''
SET @SampleSize = ISNULL(' WITH SAMPLE ' + CAST(@SamplePercent AS nvarchar(20)) + ' ' + @SampleType,N'')
WHILE @CurrentRecord <= @MaxRecord
BEGIN
SELECT
@TableName = os.TableName
,@StatName = os.StatName
FROM #OldStats os
WHERE RowNum = @CurrentRecord
SET @SQL = N'UPDATE STATISTICS ' + @TableName + ' ' + @StatName + @SampleSize
PRINT @SQL
EXEC sp_executesql @SQL
SET @CurrentRecord = @CurrentRecord + 1
END
After updating the statistics, the execution plans that use these statistics may become invalid. Ideally SQL Server should then create a new execution plan. Personally, I prefer to help SQL Server out by flushing the cache. I would recommend you do the same. Note, this clears the entire procedure cache for the server, not just the database.
DBCC FREEPROCCACHE
You should then also update the usage stats. Usage stats are the row counts stored for each index:
DBCC UPDATEUSAGE (0);
If you are not already doing so, it is highly recommended to leave the default settings of “Auto Update Statistics” and “Auto Create Statistics” ON.
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The dynamic sql fails in certain circumstances… For instance, the DB I’m working with has a table named fim.Set. The statement generated is:
UPDATE STATISTICS fim.Set _WA_Sys_00000002_3B75D760
It needs to be:
UPDATE STATISTICS fim.[Set] _WA_Sys_00000002_3B75D760
Also, I had to add brackets around the index name, as well, as some of the indexes have a hyphen in them and SQL doesn’t like that either.
You are the man, thank you for saving me hours!
Simple and after 10 websites of crap this fixed my problem with one line of code and 20 seconds of run time.
Thank you for the code.
(a minor issue: there are some html escape codes in the SQL: less-than, greater-than things)
