Saturday, 14 February 2015

All about Statistics [ Part – I ]

I got a query from one of my friends. He is asking about when to use sp_updatestats and how frequently we can use it. To answer his query, I decide to write some words related to Statistics. Hope it will be informative for all of us.

Use of Statistics Objects
We all know about the important of statistics object to make efficient execution plan by optimizer. If the statistics object is not updated properly causes poor execution plan and down the performance factors of query.

When the Statistics Object Created
When we are creating the Index the Statistics object is created automatically called the Index Statistics. These statistics will exist as long as the index exists.

Second, assuming the database option Auto Create Statistics is enabled, which it is by default, SQL Server will create single-column statistics whenever a column, which is not already the leading column in an existing index, is used in a query predicate (e.g. in the search condition of a WHERE clause, or in a JOIN condition). We refer to these as Column Statistics. We can also use the CREATE STATISTICS command to create single- and multi-column statistics manually.

When the Statistics Object Updated
When we Insert/Update/Delete records from Table objects the SQL Server automatically Insert/Update/Delete corresponding rows in the Index.

 But the Statistics is not updated like this, here we mean to say that when we Insert/Update/Delete the records from Table objects the Index updated automatically but Statistics not. Although the Auto Update Statistics option of the Database is enabled.
So we have to understand when the statistics is updated. It depends on certain volume threshold.

As data changes in our tables, the statistics - all the statistics - will be updated based on the following formula:

·         When a table with no rows gets a row
·         When 500 rows are changed to a table that is less than 500 rows
·         When 20% + 500 are changed in a table greater than 500 rows

Every time we modify a record in a table, SQL Server tracks it via the rcmodified column in a hidden system table. SQL Server 2005 tracked this information in the sys.rowsetcolumns table, In SQL Server 2008 (and later) sys.rowsetcolumns merged with sys.syshobtcolumns and became sys.sysrscols.

When we create or rebuild (not reorganize, just rebuild) an index, SQL Server generates the statistics with a FULLSCAN, i.e. it scans all the rows in the table to create a histogram that represents the distribution of data in the leading column of the index. Likewise, SQL will auto-create column statistics with a full sample.

When to Update Statistics Manually

Suppose we have table objects of 100 millions of records and SQL server is going to Update the Statistics Objects when 20% of the records of 100 million is effected by Insert/Update/Delete. So we have to wait long for Statistics Object update and result is poor exaction plan creation by SQL server. In this situation we have to update the statistics object manually.

In cases where we know data distribution in a column is "skewed", it may be necessary to update statistics manually with a full sample, or create a set of filtered statistics, in order to generate query plans of good quality.

Examining the Statistics
sp_helpstats 'tbl_CUSTOMERDTLS', 'ALL'

statistics_name                     statistics_keys
---------------                   ----------------
_WA_Sys_00000001_03BB8E22           CUSTID
_WA_Sys_00000002_03BB8E22           CUSTNAME
_WA_Sys_00000003_03BB8E22           TOTALSALES
_WA_Sys_00000004_03BB8E22           GRADE

The better approach

SELECT  [sch].[name] + '.' + [so].[name] AS [TableName] ,
        [si].[index_id] AS [Index ID] ,
        [ss].[name] AS [Statistic] ,
        STUFF(( SELECT  ', ' + [c].[name]
                FROM    [sys].[stats_columns] [sc]
                        JOIN [sys].[columns] [c]
                         ON [c].[column_id] = [sc].[column_id]
                            AND [c].[object_id] = [sc].[OBJECT_ID]
                WHERE   [sc].[object_id] = [ss].[object_id]
                        AND [sc].[stats_id] = [ss].[stats_id]
                ORDER BY [sc].[stats_column_id]
                XML PATH('')
              ), 1, 2, '') AS [ColumnsInStatistic] ,
        [ss].[auto_Created] AS [WasAutoCreated] ,
        [ss].[user_created] AS [WasUserCreated] ,
        [ss].[has_filter] AS [IsFiltered] ,
        [ss].[filter_definition] AS [FilterDefinition] ,
        [ss].[is_temporary] AS [IsTemporary]
FROM    [sys].[stats] [ss]
        JOIN [sys].[objects] AS [so] ON [ss].[object_id] = [so].[object_id]
        JOIN [sys].[schemas] AS [sch] ON [so].[schema_id] = [sch].[schema_id]
        LEFT OUTER JOIN [sys].[indexes] AS [si]
              ON [so].[object_id] = [si].[object_id]
                 AND [ss].[name] = [si].[name]
WHERE   [so].[object_id] = OBJECT_ID(N'tbl_CUSTOMERDTLS')
ORDER BY [ss].[user_created] ,
        [ss].[auto_created] ,

In next version we are trying to discuss more about it.

Hope you like it.

Posted by: MR. JOYDEEP DAS

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