SQL Statistics

Sat, 11 Apr 2009 02:53:39 +0000

In most cases it is wise to let SQL Server automatically update statistics.

To turn on automatic updates for all statistics in a database, if not already on, execute the following statement:

USE master
EXEC sp_dboption ‘MyDatabase’, ‘ auto update statistics’, ‘true’

To turn on automatic updates for all statistics on a specific table, such as Clients, execute the following statement:

USE MyDatabase
EXEC sp_autostats Clients, ‘ON’

To manually update the statistics on specific table, such as Clients, execute the following command:

USE MyDatabase
UPDATE STATISTICS Clients

Query to get the list of Indexes and Statistics and when they were updated

Sat, 11 Apr 2009 01:00:31 +0000

select
schemas.name as table_schema,
tbls.name as table_name,
i.name as index_name,
i.id as table_id,
i.indid as index_id,
i.groupid,
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 ModifiedPct,
stats_date( i.id, i.indid ) as lastStatsUpdate,
‘False’ as Processed
into ##updateStatsQueue
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 (select max(rowcnt) from sysindexes i2 where i.id = i2.id and i2.indid < 2) > 0

select * from ##updateStatsQueue

drop table ##updateStatsQueue

Query to get a list Statistics in the database (from sys.sysindexes table)

Sat, 11 Apr 2009 00:27:16 +0000

Query to get a list Statistics in the database (from sys.sysindexes table)

select name, rowcnt, rowmodctr,* from sys.sysindexes where rowcnt = 0 and name like ‘%_WA%’

All Statistics objects in the sys.sysindexes have rowcnt = 0 and its name starts from ‘_WA’

Run SP_RECOMPILE after SP_UPDATESTATS

Sat, 11 Apr 2009 00:10:55 +0000

SP_UPDATESTATS refers to work against all user-defined tables in the current database.

For the stored procedures performance you need to recompile them in order to take the new plan in to the memory, so run SP_RECOMPILE for all or required stored procedures to compare the sequence of performance.

How to do the UPDATE STATISTICS in SQL Server?

Sat, 11 Apr 2009 00:02:16 +0000

If you want to find out if an index has had its indexes updated or not, you can use the command, like this:

DBCC SHOW_STATISTICS (table_name , index_name)

This command will not only tell you when statistics were last updated, but also provide you information on the kind of statistics that has been collected for the index you are examining.

You can update the statistics using this command:
USE
EXEC sp_updatestats

How does SQL Server know when to update statistics?

Sat, 11 Apr 2009 00:00:41 +0000

SQL Server follows a very specific set of rules on when it should update the statistics of an index. Here they are:
- If the number of rows in a table are greater than 6, but less than or equal to 500, then statistics are automatically updated when there have been 500 modifications made.
- If the number of rows in the table are greater than 500, then updates are automatically made when (500 plus 20 percent of the number of rows in the table) have been modified.

If you like, you can check to see how many modifications have been made to a table, and at the same time estimate when an automatic statistics update will occur. If you go to the sysindexes table of the database in question, and look at the rowmodctr column, it will show you what the count is.

select rowmodctr,* from sys.sysindexes

From this number, you can estimate when the next automatic update of statistics will occur.

What is UPDATE STATISTICS ?

Fri, 10 Apr 2009 23:51:35 +0000

SQL Server keeps statistics about the distribution of the key values in each index and uses these statistics to determine which index(es) to use in query processing. Users can create statistics on nonindexed columns by using the CREATE STATISTICS statement.

Query optimization depends on the accuracy of the distribution steps:
- If there is significant change in the key values in the index, rerun UPDATE STATISTICS on that index.
- If a large amount of data in an indexed column has been added, changed, or removed (that is, if the distribution of key values has changed), or the table has been truncated using the TRUNCATE TABLE statement and then repopulated, use UPDATE STATISTICS.

Statistics can be created or updated on tables with computed columns only if the conditions are such that an index can be created on these columns.

Reason why Update Stats with SAMPLE may take longer then with FULLSCAN

Fri, 10 Apr 2009 21:39:43 +0000

FULLSCAN is the same as SAMPLE RATE 100 with the difference that with SAMPLE RATE, SQL Server 2005 might adjust the rate up (and use a sample rate that is sufficiently high to create useful statistics) and if the requested rate creates more values than is considered needed, it “tries to match the requested sample amount”. I’m not a 100% how it is in 2005, but in 2000 it would actually revert to FULLSCAN if the sample rate exceeded some threshold (for performance). You can check which sample rate you end up with DBCC SHOWSTATISTICS… WITH STAT_HEADER.

Now Update States with SAMPLE may take longer then with FULLSCAN, the explanation to that probably lies in how samples are gathered. Thing is, when you use a sample rate, SQL Server needs to figure out an access path (heap/cl idx/non-cl idx) and it will try to find one that is NOT physically sorted on the first column in the statistics. It does this to provide a more random sample and thus more accurate statistics. For FULLSCAN, this is not an issue so the lowest cost access path is chosen.

SQL script to find NULL statistics if they exist

Fri, 10 Apr 2009 21:28:24 +0000

SELECT ‘update statistics’ + ‘ ‘ + RTRIM(object_name(I.id)) + ‘ ‘ + RTRIM(name)

/* DATALENGTH (statblob) size,
STATS_DATE (I.id, I.indid) last_updated */
FROM
sysindexes as I

WHERE
OBJECTPROPERTY(I.id, N’IsUserTable’) = 1 AND
INDEXPROPERTY (I.id , name , ‘IsAutoStatistics’ ) = 1 AND
DATALENGTH (statblob) is null

SP_UPDATESTATS - updates statistics on tables which were modified recently

Fri, 10 Apr 2009 21:21:30 +0000

You can run sp_updatestats against the database, it will update the statistics on those tables which were modified recently.