Cardinality Estimation (SQL Server)
This article illustrates how you can assess and choose the best cardinality estimation (CE) configuration for your SQL system. Most systems benefit from the latest CE because it is the most accurate. The CE predicts how many rows your query will likely return. The cardinality prediction is used by the Query Optimizer to generate the optimal query plan. With more accurate estimations, the Query Optimizer can usually do a better job of producing a more optimal query plan.
Your application system could possibly have an important query whose plan is changed to a slower plan due to the new CE. Such a query might be like one of the following:
- An OLTP (online transaction processing) query that runs so frequently that multiple instance of it often run concurrently.
- A SELECT with substantial aggregation that runs during your OLTP business hours.
You have techniques for identifying a query that performs slower with the new CE. And you have options for how to address the performance issue.
Versions of the CE
In 1998, a major update of the CE was part of Microsoft SQL Server 7.0, for which the compatibility level was 70. Subsequent updates started with SQL Server 2014, meaning compatibility levels 120 and above. The CE updates for levels 120 and above incorporate assumptions and algorithms that work well on modern data warehousing and on OLTP workloads.
Compatibility level: You can ensure your database is at a particular level by using the following Transact-SQL code for COMPATIBILITY_LEVEL.
SELECT ServerProperty('ProductVersion'); go ALTER DATABASE <yourDatabase> SET COMPATIBILITY_LEVEL = 130; go SELECT d.name, d.compatibility_level FROM sys.databases AS d WHERE d.name = 'yourDatabase'; go
For a SQL Server database set at compatibility level 120 or above, activation of the trace flag 9481 forces the system to use the CE version 70.
Legacy CE: For a SQL Server database set at compatibility level 120 and above, the CE version 70 can be can be activated by using the at the database level by using the ALTER DATABASE SCOPED CONFIGURATION.
ALTER DATABASE SCOPED CONFIGURATION SET LEGACY_CARDINALITY_ESTIMATION = ON; go SELECT name, value FROM sys.database_scoped_configurations WHERE name = 'LEGACY_CARDINALITY_ESTIMATION';
Or starting with SQL Server 2016 SP1, the Query Hint
USE HINT ('FORCE_LEGACY_CARDINALITY_ESTIMATION').
SELECT CustomerId, OrderAddedDate FROM OrderTable WHERE OrderAddedDate >= '2016-05-01'; OPTION (USE HINT ('FORCE_LEGACY_CARDINALITY_ESTIMATION'));
Query store: Starting with SQL Server 2016, the query store is a handy tool for examining the performance of your queries. In Management Studio, in the Object Explorer under your database node, a Query Store node is displayed when the query store is enabled.
ALTER DATABASE <yourDatabase> SET QUERY_STORE = ON; go SELECT q.actual_state_desc AS [actual_state_desc-ofQueryStore], q.desired_state_desc, q.query_capture_mode_desc FROM sys.database_query_store_options AS q; go ALTER DATABASE <yourDatabase> SET QUERY_STORE CLEAR;
We recommend that you install the latest release of Management Studio and update it often.
Another option for tracking the cardinality estimation process is to use the extended event named query_optimizer_estimate_cardinality. The following T-SQL code sample runs on SQL Server. It writes a .xel file to C:\Temp\ (although you can change the path). When you open the .xel file in Management Studio, its detailed information is displayed in a user friendly manner.
DROP EVENT SESSION Test_the_CE_qoec_1 ON SERVER; go CREATE EVENT SESSION Test_the_CE_qoec_1 ON SERVER ADD EVENT sqlserver.query_optimizer_estimate_cardinality ( ACTION (sqlserver.sql_text) WHERE ( sql_text LIKE '%yourTable%' and sql_text LIKE '%SUM(%' ) ) ADD TARGET package0.asynchronous_file_target (SET filename = 'c:\temp\xe_qoec_1.xel', metadatafile = 'c:\temp\xe_qoec_1.xem' ); go ALTER EVENT SESSION Test_the_CE_qoec_1 ON SERVER STATE = START; --STOP; go
For information about extended events as tailored for SQL Database, see Extended events in SQL Database.
Steps to assess the CE version
Next are steps you can use to assess whether any of your most important queries perform less well under the latest CE. Some of the steps are performed by running a code sample presented in a preceding section.
Open Management Studio. Ensure your SQL Serverdatabase is set to the highest available compatibility level.
Perform the following preliminary steps:
Open Management Studio.
Run the T-SQL to ensure that your SQL Server database is set to the highest available compatibility level.
Ensure that your database has its
LEGACY_CARDINALITY_ESTIMATIONconfiguration turned OFF.
CLEAR your query store. Of course, ensure your query store is ON.
Run the statement:
SET NOCOUNT OFF;
Run the statement:
SET STATISTICS XML ON;
Run your important query.
In the results pane, on the Messages tab, note the actual number of rows affected.
In the results pane on the Results tab, double-click the cell that contains the statistics in XML format. A graphic query plan is displayed.
Right-click the first box in the graphic query plan, and then click Properties.
For later comparison with a different configuration, note the values for the following properties:
Estimated Number of Rows.
Estimated I/O Cost, and several similar Estimated properties that involve actual performance rather than row count predictions.
Logical Operation and Physical Operation. Parallelism is a good value.
Actual Execution Mode. Batch is a good value, better than Row.
Compare the estimated number of rows to the actual number of rows. Is the CE inaccurate by 1% (high or low), or by 10%?
SET STATISTICS XML OFF;
Run the T-SQL to decrease the compatibility level of your database by one level (such as from 130 down to 120).
Rerun all the non-preliminary steps.
Compare the CE property values from the two runs.
- Is the inaccuracy percentage under the newest CE less than under the older CE?
Finally, compare the various performance property values from the two runs.
Did your query use a different plan under the two differing CE estimations?
Did your query run slower under the latest CE?
Unless your query runs better and with a different plan under the older CE, you almost certainly want the latest CE.
However, if your query runs with a faster plan under the older CE, consider forcing the system to use the faster plan and to ignore the CE. This way you can have the latest CE on for everything, while keeping the faster plan in the one odd case.
How to activate the best query plan
Suppose that with the new CE a slower query plan is generated for your query. Here are some options you have to activate the faster plan.
You could set the compatibility level to a value lower than the latest available, for your whole database.
This activates the Legacy CE, but it makes all queries subject to the older and less accurate CE.
Further the previous level compatibility also loses excellent improvements in the query optimizer.
You could use
LEGACY_CARDINALITY_ESTIMATION to have the whole database use the older CE, or just a specific query, while retaining the improvements in the query optimizer.
For the finest control, you could force the SQL system to use the plan that was generated with the older CE during your testing. After you pin your preferred plan, you can set your whole database to use the latest compatibility level and CE. The option is elaborated next.
How to force a particular query plan
The query store gives you different ways that you can force the system to use a particular query plan:
In Management Studio, expand your Query Store node, right-click Top Resource Consuming Nodes, and then click View Top Resource Consuming Nodes. The display shows buttons labeled Force Plan and Unforce Plan.
For more information about the query store, see Monitoring Performance By Using the Query Store.
Examples of CE improvements
This section describes example queries that benefit from the enhancements implemented in the CE in recent releases. This is background information that does not call for specific action on your part.
Example A. CE understands maximum value might be higher than when statistics were last gathered
Suppose statistics were last gathered for OrderTable on 2016-04-30, when the maximum OrderAddedDate was 2016-04-30. The CE for compatibility level 120 (and for higher levels) understands that columns in OrderTable which have ascending data might have values larger than the maximum recorded by the statistics. This understanding improves the query plan for SQL SELECTs such as the following.
SELECT CustomerId, OrderAddedDate FROM OrderTable WHERE OrderAddedDate >= '2016-05-01';
Example B. CE understands that filtered predicates on the same table are often correlated
In the following SELECT we see filtered predicates on Model and ModelVariant. We intuitively understand that when Model is 'Xbox' there is a chance the ModelVariant is 'One', given that Xbox has a variant called One.
The level 120 CE understands there might be a correlation between the two columns on the same table, Model and ModelVariant. The CE makes a more accurate estimation of how many rows will be returned by the query, and the query optimizer generates a more optimal plan.
SELECT Model, Purchase_Price FROM dbo.Hardware WHERE Model = 'Xbox' AND ModelVariant = 'One';
Example C. CE no longer assumes any correlation between filtered predicates from different tables
With extense new research on modern workloads and actual business data reveal that predicate filters from different tables usually do not correlate with each other. In the following query, the CE assumes there is no correlation between s.type and r.date. Therefore the CE makes a lower estimate of the number of rows returned.
SELECT s.ticket, s.customer, r.store FROM dbo.Sales AS s CROSS JOIN dbo.Returns AS r WHERE s.ticket = r.ticket AND s.type = 'toy' AND r.date = '2016-05-11';
Monitor and Tune for Performance
Optimizing Your Query Plans with the SQL Server 2014 Cardinality Estimator
Monitoring Performance By Using the Query Store
Query Processing Architecture Guide