SQL Server Query Execution Plan Analysis
When it comes time to analyze the performance of a specific query, one of the best methods is to view the query execution plan. A query execution plan outlines how the SQL Server query optimizer actually ran (or will run) a specific query. This information if very valuable when it comes time to find out why a specific query is running slow.
There are several different ways to view a query’s execution plan. They include:
- From within Query Analyzer is an option called “Show Execution Plan” (located on the Query drop-down menu). If you turn this option on, then whenever you run a query in Query Analyzer, you will get a query execution plan (in graphical format) displayed in a separate window.
- If you want to see an execution plan, but you don’t want to run the query, you can choose the option “Display Estimated Execution Plan” (located on the Query drop-down menu). When you select this option, immediately an execution plan (in graphical format) will appear. The difference between these two (if any) is accountable to the fact that when a query is really run (not simulated, as in this option), current operations of the server are also considered. In most cases, plans created by either method will produce similar results.
- When you create a SQL Server Profiler trace, one of the events you can collect is called: MISC: Execution Plan. This information (in text form) shows the execution plan used by the query optimizer to execute the query.
- From within Query Analyzer, you can run the command SET SHOWPLAN_TEXT ON. Once you run this command, any query you execute in this Query Analyzer sessions will not be run, but a text-based version of the query plan will be displayed. If the query you are running uses temp tables, then you will have to run the command, SET STATISTICS PROFILE ON before running the query.
Of these options, I prefer using the “Show Execution Plan”, which produces a graphical output and considers current server operations. [7.0, 2000] Updated 8-5-2005
If you see any of the following in an execution plan, you should consider them warning signs and investigate them for potential performance problems. Each of them are less than ideal from a performance perspective.
- Index or table scans: May indicate a need for better or additional indexes.
- Bookmark Lookups: Consider changing the current clustered index, consider using a covering index, limit the number of columns in the SELECT statement.
- Filter: Remove any functions in the WHERE clause, don’t include wiews in your Transact-SQL code, may need additional indexes.
- Sort: Does the data really need to be sorted? Can an index be used to avoid sorting? Can sorting be done at the client more efficiently?
It is not always possible to avoid these, but the more you can avoid them, the faster query performance will be. [7.0, 2000, 2005] Updated 8-5-2005
If you have a stored procedure, or other batch Transact-SQL code that uses temp tables, you cannot use the “Display Estimated Execution Plan” option in the Query Analyzer or Management Studio to evaluate it. Instead, you must actually run the stored procedure or batch code. This is because when a query is run using the “Display Estimated Execution Plan” option, it is not really run, and temp tables are not created. Since they are not created, any references to them in the code will fail, which prevents an estimated execution plan from being created.
On the other hand, if you use a table variable instead of a temp table, you can use the “Display Estimated Execution Plan” option [7.0, 2000, 2005] Updated 8-5-2005
If you have a very complex query you are analyzing in Query Analyzer or Management Studio as a graphical query execution plan, the resulting plan can be very difficult to view and analyze. You may find it easier to break down the query into its logical components, analyzing each component separately. [7.0, 2000, 2005] Updated 8-5-2005
The results of a graphical query execution plan are not always easy to read and interpret. Keep the following in mind when viewing a graphical execution plan:
- In very complex query plans, the plan is divided into many parts, with each part listed one on top of the other on the screen. Each part represents a separate process or step that the query optimizer has to perform in order to get to the final results.
- Each of the execution plan steps is often broken down into smaller sub-steps. Unfortunately, they are displayed on the screen from right to left. This means you must scroll to the far right of the graphical query plan to see where each step starts.
- Each of the sub-steps and steps is connected by an arrow, showing the path (order) taken of the query when it was executed.
- Eventually, all of the parts come together at the top left side of the screen.
- If you move your cursor above any of the steps or sub-steps, a pop-up windows is displayed, providing more detailed information about this particular step or sub-step.
- If you move your cursor over any of the arrows connecting the steps and sub-steps, you see a pop-up window showing how many records are being moved from one step or sub-step to another step or sub-step.
[7.0, 2000, 2005] Updated 8-5-2005