To be able to execute queries, the SQL Server Database Engine 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
SELECTstatement references three tables, the database server could first access
TableA, use the data from
TableAto extract matching rows from
TableB, and then use the data from
TableBto extract data from
TableC. The other sequences in which the database server could access the tables are:
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
ORDER BYclause, and how to filter data, for example when using a
SQL Server Management Studio has three options to display execution plans:
- The Estimated Execution Plan is the compiled plan, as produced by the Query Optimizer based on estimations. This is the query plan that is stored in the plan cache.
- The Actual Execution Plan is the compiled plan plus its execution context. It becomes available after the query execution has completed. This includes actual runtime information such as execution warnings, or in newer versions of the Database Engine, the elapsed and CPU time used during execution.
- The Live Query Statistics is the compiled plan plus its execution context. It is available for in-flight query executions, and updated every second. This includes runtime information such as the actual number of rows flowing through the operators, elapsed time, and the estimated query progress.
In This Section
Monitor and Tune for Performance
Performance Monitoring and Tuning Tools
Query Processing Architecture Guide
Live Query Statistics
Monitoring Performance by Using the Query Store
Showplan Logical and Physical Operators Reference