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The query optimizer uses statistics to create query plans that improve query performance. For most queries, the query optimizer already generates the necessary statistics for a high quality query plan; in a few cases, you need to create additional statistics or modify the query design for best results. This topic discusses statistics concepts and provides guidelines for using query optimization statistics effectively.
The query optimizer already creates statistics in the following ways:
The query optimizer creates statistics for indexes on tables or views when the index is created. These statistics are created on the key columns of the index. If the index is a filtered index, the query optimizer creates filtered statistics on the same subset of rows specified for the filtered index. For more information about filtered indexes, see Create Filtered Indexes and CREATE INDEX (Transact-SQL).
The query optimizer creates statistics for single columns in query predicates when AUTO_CREATE_STATISTICS is on.
For most queries, these two methods for creating statistics ensure a high-quality query plan; in a few cases, you can improve query plans by creating additional statistics with the CREATE STATISTICS statement. These additional statistics can capture statistical correlations that the query optimizer does not account for when it creates statistics for indexes or single columns. Your application might have additional statistical correlations in the table data that, if calculated into a statistics object, could enable the query optimizer to improve query plans. For example, filtered statistics on a subset of data rows or multicolumn statistics on query predicate columns might improve the query plan.
When creating statistics with the CREATE STATISTICS statement, we recommend keeping the AUTO_CREATE_STATISTICS option on so that the query optimizer continues to routinely create single-column statistics for query predicate columns. For more information about query predicates, see Search Condition (Transact-SQL).
Consider creating statistics with the CREATE STATISTICS statement when any of the following applies:
The Database Engine Tuning Advisor suggests creating statistics.
The query predicate contains multiple correlated columns that are not already in the same index.
The query selects from a subset of data.
The query has missing statistics.
Query Predicate Contains Multiple Correlated Columns
When a query predicate contains multiple columns that have cross-column relationships and dependencies, statistics on the multiple columns might improve the query plan. Statistics on multiple columns contain cross-column correlation statistics, called densities, that are not available in single-column statistics. Densities can improve cardinality estimates when query results depend on data relationships among multiple columns.
If the columns are already in the same index, the multicolumn statistics object already exists and it is not necessary to create it manually. If the columns are not already in the same index, you can create multicolumn statistics by creating an index on the columns or by using the CREATE STATISTICS statement. It requires more system resources to maintain an index than a statistics object. If the application does not require the multicolumn index, you can economize on system resources by creating the statistics object without creating the index.
When creating multicolumn statistics, the order of the columns in the statistics object definition affects the effectiveness of densities for making cardinality estimates. The statistics object stores densities for each prefix of key columns in the statistics object definition. For more information about densities, see DBCC SHOW_STATISTICS (Transact-SQL).
To create densities that are useful for cardinality estimates, the columns in the query predicate must match one of the prefixes of columns in the statistics object definition. For example, the following creates a multicolumn statistics object on the columns LastName, MiddleName, and FirstName.
USE AdventureWorks2012; GO IF EXISTS (SELECT name FROM sys.stats WHERE name = 'LastFirst' AND object_ID = OBJECT_ID ('Person.Person')) DROP STATISTICS Person.Person.LastFirst; GO CREATE STATISTICS LastFirst ON Person.Person (LastName, MiddleName, FirstName); GO
In this example, the statistics object LastFirst has densities for the following column prefixes: (LastName), (LastName, MiddleName), and (LastName, MiddleName, FirstName). The density is not available for (LastName, FirstName). If the query uses LastName and FirstName without using MiddleName, the density is not available for cardinality estimates.
Query Selects from a Subset of Data
When the query optimizer creates statistics for single columns and indexes, it creates the statistics for the values in all rows. When queries select from a subset of rows, and that subset of rows has a unique data distribution, filtered statistics can improve query plans. You can create filtered statistics by using the CREATE STATISTICS statement with the WHERE clause to define the filter predicate expression.
For example, using AdventureWorks2012, each product in the Production.Product table belongs to one of four categories in the Production.ProductCategory table: Bikes, Components, Clothing, and Accessories. Each of the categories has a different data distribution for weight: bike weights range from 13.77 to 30.0, component weights range from 2.12 to 1050.00 with some NULL values, clothing weights are all NULL, and accessory weights are also NULL.
Using Bikes as an example, filtered statistics on all bike weights will provide more accurate statistics to the query optimizer and can improve the query plan quality compared with full-table statistics or nonexistent statistics on the Weight column. The bike weight column is a good candidate for filtered statistics but not necessarily a good candidate for a filtered index if the number of weight lookups is relatively small. The performance gain for lookups that a filtered index provides might not outweigh the additional maintenance and storage cost for adding a filtered index to the database.
The following statement creates the BikeWeights filtered statistics on all of the subcategories for Bikes. The filtered predicate expression defines bikes by enumerating all of the bike subcategories with the comparison Production.ProductSubcategoryID IN (1,2,3). The predicate cannot use the Bikes category name because it is stored in the Production.ProductCategory table, and all columns in the filter expression must be in the same table.
USE AdventureWorks2012; GO IF EXISTS ( SELECT name FROM sys.stats WHERE name = 'BikeWeights' AND object_ID = OBJECT_ID ('Production.Product')) DROP STATISTICS Production.Product.BikeWeights; GO CREATE STATISTICS BikeWeights ON Production.Product (Weight) WHERE ProductSubcategoryID IN (1,2,3); GO
The query optimizer can use the BikeWeights filtered statistics to improve the query plan for the following query that selects all of the bikes that weigh more than 25.
SELECT P.Weight AS Weight, S.Name AS BikeName FROM Production.Product AS P JOIN Production.ProductSubcategory AS S ON P.ProductSubcategoryID = S.ProductSubcategoryID WHERE P.ProductSubcategoryID IN (1,2,3) AND P.Weight > 25 ORDER BY P.Weight; GO
Query Identifies Missing Statistics
If an error or other event prevents the query optimizer from creating statistics, the query optimizer creates the query plan without using statistics. The query optimizer marks the statistics as missing and attempts to regenerate the statistics the next time the query is executed.
Missing statistics are indicated as warnings (table name in red text) when the execution plan of a query is graphically displayed using SQL Server Management Studio. Additionally, monitoring the Missing Column Statistics event class by using SQL Server Profiler indicates when statistics are missing. For more information, see Errors and Warnings Event Category (Database Engine).
If statistics are missing, perform the following steps:
Verify that AUTO_CREATE_STATISTICS and AUTO_UPDATE_STATISTICS are on.
Verify that the database is not read-only. If the database is read-only, the query optimizer cannot save statistics.
Create the missing statistics by using the CREATE STATISTICS statement.
When statistics on a read-only database or read-only snapshot are missing or stale, the Database Engine creates and maintains temporary statistics in tempdb. When the Database Engine creates temporary statistics, the statistics name is appended with the suffix _readonly_database_statistic to differentiate the temporary statistics from the permanent statistics. The suffix _readonly_database_statistic is reserved for statistics generated by SQL Server. Scripts for the temporary statistics can be created and reproduced on a read-write database. When scripted, Management Studio changes the suffix of the statistics name from _readonly_database_statistic to _readonly_database_statistic_scripted.
Only SQL Server can create and update temporary statistics. However, you can delete temporary statistics and monitor statistics properties using the same tools that you use for permanent statistics:
Delete temporary statistics using the DROP STATISTICS (Transact-SQL) statement.
Monitor statistics using the sys.stats and sys.stats_columns catalog views. sys_stats includes the is_temporary column, to indicate which statistics are permanent and which are temporary.
Because temporary statistics are stored in tempdb, a restart of the SQL Server service causes all temporary statistics to disappear.
The query optimizer determines when statistics might be out-of-date and then updates them when they are needed for a query plan. In some cases you can improve the query plan and therefore improve query performance by updating statistics more frequently than occur when AUTO_UPDATE_STATISTICS is on. You can update statistics with the UPDATE STATISTICS statement or the stored procedure sp_updatestats.
Updating statistics ensures that queries compile with up-to-date statistics. However, updating statistics causes queries to recompile. We recommend not updating statistics too frequently because there is a performance tradeoff between improving query plans and the time it takes to recompile queries. The specific tradeoffs depend on your application.
When updating statistics with UPDATE STATISTICS or sp_updatestats, we recommend keeping AUTO_UPDATE_STATISTICS set to ON so that the query optimizer continues to routinely update statistics. For more information about how to update statistics on a column, an index, a table, or an indexed view, see UPDATE STATISTICS (Transact-SQL). For information about how to update statistics for all user-defined and internal tables in the database, see the stored procedure sp_updatestats (Transact-SQL).
To determine when statistics were last updated, use the STATS_DATE function.
Consider updating statistics for the following conditions:
Query execution times are slow.
Insert operations occur on ascending or descending key columns.
After maintenance operations.
Query Execution Times Are Slow
If query response times are slow or unpredictable, ensure that queries have up-to-date statistics before performing additional troubleshooting steps.
Insert Operations Occur on Ascending or Descending Key Columns
Statistics on ascending or descending key columns, such as IDENTITY or real-time timestamp columns, might require more frequent statistics updates than the query optimizer performs. Insert operations append new values to ascending or descending columns. The number of rows added might be too small to trigger a statistics update. If statistics are not up-to-date and queries select from the most recently added rows, the current statistics will not have cardinality estimates for these new values. This can result in inaccurate cardinality estimates and slow query performance.
For example, a query that selects from the most recent sales order dates will have inaccurate cardinality estimates if the statistics are not updated to include cardinality estimates for the most recent sales order dates.
After Maintenance Operations
Consider updating statistics after performing maintenance procedures that change the distribution of data, such as truncating a table or performing a bulk insert of a large percentage of the rows. This can avoid future delays in query processing while queries wait for automatic statistics updates.
Operations such as rebuilding, defragmenting, or reorganizing an index do not change the distribution of data. Therefore, you do not need to update statistics after performing ALTER INDEX REBUILD, DBCC REINDEX, DBCC INDEXDEFRAG, or ALTER INDEX REORGANIZE operations. The query optimizer updates statistics when you rebuild an index on a table or view with ALTER INDEX REBUILD or DBCC DBREINDEX, however; this statistics update is a byproduct of re-creating the index. The query optimizer does not update statistics after DBCC INDEXDEFRAG or ALTER INDEX REORGANIZE operations.
Certain query implementations, such as local variables and complex expressions in the query predicate, can lead to suboptimal query plans. Following query design guidelines for using statistics effectively can help to avoid this. For more information about query predicates, see Search Condition (Transact-SQL).
You can improve query plans by applying query design guidelines that use statistics effectively to improve cardinality estimates for expressions, variables, and functions used in query predicates. When the query optimizer does not know the value of an expression, variable, or function, it does not know which value to lookup in the histogram and therefore cannot retrieve the best cardinality estimate from the histogram. Instead, the query optimizer bases the cardinality estimate on the average number of rows per distinct value for all of the sampled rows in the histogram. This leads to suboptimal cardinality estimates and can hurt query performance.
The following guidelines describe how to write queries to improve query plans by improving cardinality estimates.
Improving Cardinality Estimates for Expressions
To improve cardinality estimates for expressions, follow these guidelines:
Whenever possible, simplify expressions with constants in them. The query optimizer does not evaluate all functions and expressions containing constants prior to determining cardinality estimates. For example, simplify the expression ABS(-100) to 100.
If the expression uses multiple variables, consider creating a computed column for the expression and then create statistics or an index on the computed column. For example, the query predicate WHERE PRICE + Tax > 100 might have a better cardinality estimate if you create a computed column for the expression Price + Tax.
Improving Cardinality Estimates for Variables and Functions
To improve the cardinality estimates for variables and functions, follow these guidelines:
If the query predicate uses a local variable, consider rewriting the query to use a parameter instead of a local variable. The value of a local variable is not known when the query optimizer creates the query execution plan. When a query uses a parameter, the query optimizer uses the cardinality estimate for the first actual parameter value that is passed to the stored procedure.
Consider using a standard table or temporary table to hold the results of multi-statement table-valued functions. The query optimizer does not create statistics for multi-statement table-valued functions. With this approach the query optimizer can create statistics on the table columns and use them to create a better query plan.
Consider using a standard table or temporary table as a replacement for table variables. The query optimizer does not create statistics for table variables. With this approach the query optimizer can create statistics on the table columns and use them to create a better query plan. There are tradeoffs in determining whether to use a temporary table or a table variable; Table variables used in stored procedures cause fewer recompilations of the stored procedure than temporary tables. Depending on the application, using a temporary table instead of a table variable might not improve performance.
If a stored procedure contains a query that uses a passed-in parameter, avoid changing the parameter value within the stored procedure before using it in the query. The cardinality estimates for the query are based on the passed-in parameter value and not the updated value. To avoid changing the parameter value, you can rewrite the query to use two stored procedures.
For example, the following stored procedure Sales.GetRecentSales changes the value of the parameter @date when @date is NULL.
USE AdventureWorks2012; GO IF OBJECT_ID ( 'Sales.GetRecentSales', 'P') IS NOT NULL DROP PROCEDURE Sales.GetRecentSales; GO CREATE PROCEDURE Sales.GetRecentSales (@date datetime) AS BEGIN IF @date is NULL SET @date = DATEADD(MONTH, -3, (SELECT MAX(ORDERDATE) FROM Sales.SalesOrderHeader)) SELECT * FROM Sales.SalesOrderHeader h, Sales.SalesOrderDetail d WHERE h.SalesOrderID = d.SalesOrderID AND h.OrderDate > @date END GO
If the first call to the stored procedure Sales.GetRecentSales passes a NULL for the @date parameter, the query optimizer will compile the stored procedure with the cardinality estimate for @date = NULL even though the query predicate is not called with @date = NULL. This cardinality estimate might be significantly different than the number of rows in the actual query result. As a result, the query optimizer might choose a suboptimal query plan. To help avoid this, you can rewrite the stored procedure into two procedures as follows:
USE AdventureWorks2012; GO IF OBJECT_ID ( 'Sales.GetNullRecentSales', 'P') IS NOT NULL DROP PROCEDURE Sales.GetNullRecentSales; GO CREATE PROCEDURE Sales.GetNullRecentSales (@date datetime) AS BEGIN IF @date is NULL SET @date = DATEADD(MONTH, -3, (SELECT MAX(ORDERDATE) FROM Sales.SalesOrderHeader)) EXEC Sales.GetNonNullRecentSales @date; END GO IF OBJECT_ID ( 'Sales.GetNonNullRecentSales', 'P') IS NOT NULL DROP PROCEDURE Sales.GetNonNullRecentSales; GO CREATE PROCEDURE Sales.GetNonNullRecentSales (@date datetime) AS BEGIN SELECT * FROM Sales.SalesOrderHeader h, Sales.SalesOrderDetail d WHERE h.SalesOrderID = d.SalesOrderID AND h.OrderDate > @date END GO
Improving Cardinality Estimates with Query Hints
To improve cardinality estimates for local variables, you can use the OPTIMIZE FOR or OPTIMIZE FOR UNKNOWN query hints with RECOMPILE. For more information, see Query Hints (Transact-SQL).
For some applications, recompiling the query each time it executes might take too much time. The OPTIMIZER FOR query hint can help even if you don't use the RECOMPILE option. For example, you could add an OPTIMIZER FOR option to the stored procedure Sales.GetRecentSales to specify a specific date. The following example adds the OPTIMIZE FOR option to the Sales.GetRecentSales procedure.
USE AdventureWorks2012; GO IF OBJECT_ID ( 'Sales.GetRecentSales', 'P') IS NOT NULL DROP PROCEDURE Sales.GetRecentSales; GO CREATE PROCEDURE Sales.GetRecentSales (@date datetime) AS BEGIN IF @date is NULL SET @date = DATEADD(MONTH, -3, (SELECT MAX(ORDERDATE) FROM Sales.SalesOrderHeader)) SELECT * FROM Sales.SalesOrderHeader h, Sales.SalesOrderDetail d WHERE h.SalesOrderID = d.SalesOrderID AND h.OrderDate > @date OPTION ( OPTIMIZE FOR ( @date = '2004-05-01 00:00:00.000')) END; GO
Improving Cardinality Estimates with Plan Guides
For some applications, query design guidelines might not apply because you cannot change the query or using the RECOMPILE query hint might be cause too many recompiles. You can use plan guides to specify other hints, such as USE PLAN, to control the behavior of the query while investigating application changes with the application vendor. For more information about plan guides, see Plan Guides.