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What Is Statistics In SQL Server



Statistics is a histogram displaying the distribution of values in the first column of statistics.

  1. The query optimizer uses statistics to create query plans that improve query performance
  2. Statistics for query optimization are objects that contain statistical information about the distribution of values in one or more columns of a table or indexed view.
  3. The query optimizer uses these statistics to estimate the cardinality, or number of rows, in the query result
  4. These cardinality estimates enable the query optimizer to create a high-quality query plan. For example, the query optimizer could use cardinality estimates to choose the index seek operator instead of the more resource-intensive index scan operator.
  5. Each statistics object is created on a list of one or more table columns and includes a histogram displaying the distribution of values in the first column
  6. Statistics objects on multiple columns also store statistical information about the correlation of values among the columns
  7. These correlation statistics, or densities, are derived from the number of distinct rows of column values
  8. You need to create additional statistics or modify the query design for best results
 Filtered Statistics

9)      Filtered statistics can improve query performance for queries that select from well-defined subsets of data.
10)  Filtered statistics use a filter predicate to select the subset of data that is included in the statistics.
11)  Well-designed filtered statistics can improve the query execution plan compared with full-table statistics
 
Example: 
               CREATE STATISTICS ContactPromotion1
                       ON Person.Person (BusinessEntityID,LastName, EmailPromotion)
                WHEREEmailPromotion = 2
            WITH SAMPLE 50 PERCENT;
 


Three options that you can set that affect when and how statistics are created and updated, these options are set at the database level only.

·         AUTO_CREATE_STATISTICS Option
·         AUTO_UPDATE_STATISTICS Option
·         AUTO_UPDATE_STATISTICS_ASYNC

AUTO_CREATE_STATISTICS:
1)      When the automatic create statistics option “ AUTO_CREATE_STATISTICS” is on, the query optimizer creates statistics on individual columns in the query predicate, as necessary, to improve cardinality estimates for the query plan.
2)      These single-column statistics are created on columns that do not already have a histogram in an existing statistics object.
3)      This option does not determine whether statistics get created for indexes.
4)      This option also does not generate filtered statistics.
5)       It applies strictly to single-column statistics for the full table

                         Query predicate = where condition
                                        When the query optimizer creates statistics as a result of using the AUTO_CREATE_STATISTICS option, the statistics name starts with “ _WA” and column “auto created” in “sys.stats” table is 1  You can use the following query to determine if the query optimizer has created statistics for a query predicate column.

            SELECT OBJECT_NAME(s.object_id) AS object_name,
        COL_NAME(sc.object_id, sc.column_id) AS column_name,
          s.name AS statistics_name
          FROM sys.stats AS s JOIN sys.stats_columns AS sc
        ON s.stats_id = sc.stats_id AND s.object_id = sc.object_id
            WHERE s.name like '_WA%' and auto_created=1
              ORDER BY s.name;

AUTO_UPDATE_STATISTICS: 
1)      When the option “ AUTO_UPDATE_STATISTICS” is on, the query optimizer determines when statistics might be out-of-date and then updates them when they are used by a query.
2)      Statistics become out-of-date after insert, update, delete, or merge operations change the data distribution in the table or indexed view.
3)      The query optimizer determines when statistics might be out-of-date by counting the number of data modifications (DML) since the last statistics update and comparing the number of modifications to a threshold. The threshold is based on the number of rows in the table or indexed view.
4)      The query optimizer checks for out-of-date statistics before compiling a query and before executing a cached query plan
5)      Before compiling a query, the query optimizer uses the columns, tables, and indexed views in the query predicate to determine which statistics might be out-of-date.
6)      Before executing a cached query plan, the Database Engine verifies that the query plan references up-to-date statistics(Statistics should be updated before query get executed).
7)      This option applies to statistics objects created for( indexes, single-columns in query predicates{statistics created by auto create statistics}, and statistics created with the CREATE STATISTICS statement and filtered statistics)

AUTO_UPDATE_STATISTICS_ASYNC: The asynchronous statistics update option, determines whether the query optimizer uses synchronous or asynchronous statistics updates. By default, the asynchronous statistics update option is off, and the query optimizer updates statistics synchronously.
1)      This option applies to statistics objects created for indexes, single columns in query predicates, and statistics created with the CREATE STATISTICSstatement.
2)      With asynchronous statistics updates, queries compile with existing statistics even if the existing statistics are out-of-date.
3)      The query optimizer could choose a suboptimal query plan if statistics are out-of-date when the query compiles (query will run and later update statistics).
4)      Queries that compile after the asynchronous updates have completed will benefit from using the updated statistics.
5)      Benefit: Your application has experienced client request time outs caused by one or more queries waiting for updated statistics. In some cases, waiting for synchronous statistics (AUTO_UPDATE_STATISTICS) could cause applications with aggressive time outs to fail.

Thanks to Tinnu Babu T Paily for drafting this post

References: 


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