Skip to content

execution-plans.md

Latest commit

 

History

History
65 lines (56 loc) · 6.04 KB

execution-plans.md

File metadata and controls

65 lines (56 loc) · 6.04 KB
title Execution Plans | Microsoft Docs
ms.custom
ms.date 03/01/2020
ms.prod sql
ms.reviewer
ms.technology performance
ms.topic conceptual
helpviewer_keywords
query plans [SQL Server]
execution plans [SQL Server]
execution plan [SQL Server]
query plan [SQL Server]
query execution plans
ms.assetid 07f8f594-75b4-4591-8c29-d63811d7753f
author pmasl
ms.author pelopes
manager amitban

Execution Plans

[!INCLUDEappliesto-ss-xxxx-xxxx-xxx-md]

To be able to execute queries, the [!INCLUDEssDEnoversion] must analyze the statement to determine the most efficient way to access the required data. This analysis is handled by a component called the Query Optimizer. The input to the Query Optimizer consists of the query, the database schema (table and index definitions), and the database statistics. The output of the Query Optimizer is a query execution plan, sometimes referred to as a query plan, or execution plan.

A query execution plan is a definition of the following:

  • The sequence in which the source tables are accessed.
    Typically, there are many sequences in which the database server can access the base tables to build the result set. For example, if a SELECT statement references three tables, the database server could first access TableA, use the data from TableA to extract matching rows from TableB, and then use the data from TableB to extract data from TableC. The other sequences in which the database server could access the tables are:
    TableC, TableB, TableA, or
    TableB, TableA, TableC, or
    TableB, TableC, TableA, or
    TableC, TableA, TableB

  • The methods used to extract data from each table.
    Generally, there are different methods for accessing the data in each table. If only a few rows with specific key values are required, the database server can use an index. If all the rows in the table are required, the database server can ignore the indexes and perform a table scan. If all the rows in a table are required but there is an index whose key columns are in an ORDER BY, performing an index scan instead of a table scan may save a separate sort of the result set. If a table is very small, table scans may be the most efficient method for almost all access to the table.

  • The methods used to compute calculations, and how to filter, aggregate, and sort data from each table.
    As data is accessed from tables, there are different methods to perform calculations over data such as computing scalar values, and to aggregate and sort data as defined in the query text, for example when using a GROUP BY or ORDER BY clause, and how to filter data, for example when using a WHERE or HAVING clause.

Note

[!INCLUDEssManStudioFull] has three options to display execution plans:

  • The Estimated Execution Plan, which is the compiled plan, as produced by the Query Optimizer based on estimations.
  • The Actual Execution Plan, which is the same as the compiled plan plus its execution context. This includes actual runtime information that is available after the execution completes, such as execution warnings, or in newer versions of the [!INCLUDEssde_md], the elapsed and CPU time used during execution.
  • The Live Query Statistics, which is the same as the compiled plan plus its execution context. This includes runtime information during execution progress, and is updated every second. Runtime information includes for example the actual number of rows flowing through the operators.

Tip

For more information on query processing and query execution plans, see the sections Optimizing SELECT statements and Execution Plan Caching and Reuse of the Query Processing Architecture Guide.

In This Section

Query Profiling Infrastructure
Display and Save Execution Plans
Compare and Analyze Execution Plans
Plan Guides

See Also

Monitor and Tune for Performance
Performance Monitoring and Tuning Tools
Query Processing Architecture Guide
Live Query Statistics
Activity Monitor
Monitoring Performance by Using the Query Store
sys.dm_exec_query_statistics_xml
sys.dm_exec_query_profiles
Trace flags
Showplan Logical and Physical Operators Reference