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Archive for May 4th, 2012

Shedding light on SQL Server Statistics and Auto Update Statistics

Posted by Sudarshan Narasimhan on May 4, 2012


SQL Server collects statistical information about indexes and column data in the database. These statistics play a very important role in coming up with an efficient query plan. Statistical information is used by the Query Optimizer when it decides to choose Seek vs. Scan or using Index A vs. Index B, etc. So it’s important as a DBA to pay careful attention to statistics and making sure they are updated and maintained well.

Just like Chandler Bing’s job is “Statistical analysis and data reconfiguration”, its a good DBA’s job to take care of maintaining the statistics Smile.


SQL Server 2005 statistics features allow you to:

  • Implicitly create and update statistics with the default sampling rate (in the SELECT, INSERT, DELETE, and UPDATE commands, use of a column in a query condition such as a WHERE or JOIN clause causes statistics to be created or updated if necessary when automatic statistics creation and update is enabled)
  • Manually create and update statistics with any desired sampling rate, and drop statistics (CREATE STATISTICS, UPDATE STATISTICS, DROP STATISTICS, CREATE INDEX, DROP INDEX)
  • Manually create statistics in bulk for all columns of all tables in a database (sp_createstats)
  • Manually update all existing statistics in the database (sp_updatestats)
  • List statistics objects that exist for a table or database (sp_helpstats, catalog views sys.stats, sys.stats_columns)
  • Display descriptive information about statisticsobjects (DBCC SHOW_STATISTICS)
  • Enable and disable automatic creation and update of statistics database-wide or for a specific table or statistics object (ALTER DATABASE options AUTO_CREATE_STATISTICS and AUTO_UPDATE_STATISTICS, sp_autostats, and the NORECOMPUTE option on CREATE STATISTICS and UPDATE STATISTICS)
  • Enable and disable asynchronous automatic update ofstatistics (ALTER DATABASE option AUTO_UPDATE_STATISTICS_ASYNC

Behaviour change from SQL 2005 onwards

In SQL Server 2000, statistics update was determined by the number of row changes. Now, changes are tracked at the column level, and auto update of statistics can now be avoided on columns which have not changed enough to warrant statistics update.  This is a behaviour change between SQL 2000 and SQL 2005 onwards when it comes to auto update statistics. SQL Server 2005 and onwards determines whether to update statistics based on the colmodctrs (column modification counters) value.

A statistics object is considered out of date in the following cases:

1. The table size has gone from 0 to > 0 rows.
2. The number of rows in the table when the statistics were gathered was 500 or less, and the colmodctr of the leading column of the statistics object has changed by more than 500 since then.
3. The table had more than 500 rows when the statistics were gathered, and the colmodctr of the leading column of the statistics object has changed by more than 500 + 20% of the number of rows in the table when the statistics were gathered.
4. If the statistics object is defined on a temporary table, it is out of date as above, except that there is an additional threshold Recomputation after insertion of 6 rows.


Q: How is the COLMODCTR maintained by SQL Server?
Ans: The colmodctr values that SQL Server keeps track of are continually modified as the data in the table changes. Depending on the amount of INSERT/UPDATE/DELETE/TRUNCATE/BULK OPS that are occurring on the table, this counter value keeps changing. The below table, describes what the effect on colmodctr value are for each operation.

Cc293623.table_C05621961_2(en-us,TechNet.10).jpg
Reference: http://technet.microsoft.com/en-us/library/cc293623.aspx

 

Controlling STATISTICS Behaviour

You have options like AUTO_UPDATE_STATISTICS, AUTO_CREATE_STATISTICS and AUTO_UPDATE_STATISTICS_ASYNC etc, which are all database level options. So you need to configure this per each database. You can find if your user databases have these options by using sp_helpdb and looking at the status column or by using a query like below.

select name as [DB_NAME], is_auto_create_stats_on, is_auto_update_stats_on, is_auto_update_stats_async_on 
from sys.databases

How to find out which indexes or statistics needs to be updates?

You can use the following query on any SQL 2005+ instance to find out the % of rows modified and based on this decide if any indexes need to be rebuilt or statistics on the indexes need to be updated.

select
schemas.name as table_schema,
tbls.name as Object_name,
i.id as Object_id,
i.name as index_name,
i.indid as index_id,
i.rowmodctr as modifiedRows,
(select max(rowcnt) from sysindexes i2 where i.id = i2.id and i2.indid < 2) as rowcnt,
convert(DECIMAL(18,8), convert(DECIMAL(18,8),i.rowmodctr) / convert(DECIMAL(18,8),(select max(rowcnt) from sysindexes i2 where i.id = i2.id and i2.indid < 2))) as ModifiedPercent,
stats_date( i.id, i.indid ) as lastStatsUpdateTime
from sysindexes i
inner join sysobjects tbls on i.id = tbls.id
inner join sysusers schemas on tbls.uid = schemas.uid
inner join information_schema.tables tl
on tbls.name = tl.table_name
and schemas.name = tl.table_schema
and tl.table_type='BASE TABLE'
where 0 < i.indid and i.indid < 255
and table_schema <> 'sys'
and i.rowmodctr <> 0
and i.status not in (8388704,8388672)
and (select max(rowcnt) from sysindexes i2 where i.id = i2.id and i2.indid < 2) > 0
order by modifiedRows desc

Updating Statistics on all the table in any given database

I often get this often as to how you update all the tables in a database. You can use a script like below to achieve this.

Disclaimer: Do not run this unless you know its implications on a production server. Statistics update on all the tables will use CPU resources and depending on the size of the table take its own time.

use <YOUR_DB_NAME>
go
select identity(int,1,1) as rownum,table_name into table_count from information_schema.tables where table_type='base table'
declare @count int,@stmt varchar (255),@maxcount int,@tblname varchar(50)
set @count=1
select @maxcount=count(*) from table_count
while @count < @maxcount+1
begin 
      select @tblname=table_name from table_count where rownum=@count
      set @stmt = 'UPDATE STATISTICS '+ '[' +@tblname+ ']' + ' WITH FULLSCAN'
      PRINT ('Updating statistics for table :'+@tblname)
      EXEC(@stmt)
      PRINT ('Finished Updating statistics for table :'+@tblname)
      print ''
      set @count=@count+1
      set @stmt=''
End
drop table table_count

Some tips on Statistics

1. Table variables do not have statistics at all.

Table variables are meant for operations on a small number of rows, a few thousand rows at max. This is a good scenario where you need to think about temporary tables (#tbl), because unlike table variables, temp tables can have indexes created on them, which means they can have statistics.

2. Multi-Statement Table Value Functions (TVF’s) also do not have any statistics

So if you have a complex query logic implemented in a function in SQL Server, think again! This function does not have any statistical information present, so the SQL optimizer must guess the size of the results returned. The reason for this is a multi-statement TVF returns you a TABLE as an output and table does not have any statistics on it.

3. You can find out from the Execution Plan aka SET STATISTICS PROFILE statement if any statistics would help a particular query

When you enable STATISTICS PROFILE ON and execute any query/batch it displays the execution plan. In this output look for the column called “Warnings”. During the course of compiling the plan, if the SQL Server optimizer felt that some statistics on column A would have helped the query, it displays this warning in the execution plan as “NO STATS”. If you see any such warning, consider creating some column statistics or indexes on the particular object in the row.

4. Avoid creating indexes on very frequently updated columns as the statistics also will have to keep up with the amount of data modifications.

5. Viewing Statistics

You can use DBCC SHOW_STATISICS (‘tablename’ , ‘index name’) to view the statistics on any given index’/column stats along with the histogram. The system DMV sys.stats stores information on each statistics available in a particular database.

Any statistics having the name prefixed as _WA_Sys_ is a auto-created statistics, which means SQL Server itself created them. User created statistics will have a given name or have the index name, e.g. PK_TBL1

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