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DROP … FROM PLANCACHE

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Forces a plan to be optimized and generated from scratch the next time it is run.

Syntax

DROP [<plan_id>] FROM PLANCACHE ON NODE <node_id>


DROP PLAN FROM PLANCACHE [ON AGGREGATORS] FOR QUERY <query_text>

Remarks

  • <plan_id> - ID of the query plan to remove from memory and the disk. You can find the plan_id by running SHOW PLANCACHE.

  • The DROP PLAN FROM PLANCACHE syntax can be used to drop plans for specific queries.

  • <node_id> - the ID of the aggregator node to remove from memory and the disk. You can find the node_id for an aggregator by running SHOW AGGREGATORS.

  • This command will only drop plans stored on the node where it is run.

  • Dropping plan cache entries on an aggregator node will not drop the corresponding leaf-only plans from the leaf plan caches.

  • Avoid running this command frequently since all queries are recompiled, resulting in temporary decrease in workload performance.

Example

DROP 123 FROM PLANCACHE ON NODE 2;
Query OK, 0 rows affected (0.00 sec)
DROP PLAN FROM PLANCACHE FOR QUERY SELECT * FROM testtable;
DROP PLAN FROM PLANCACHE ON AGGREGATORS FOR QUERY SELECT * FROM testtable;

Use Cases

Example 1: DBA app tuning

A DBA wants to create some new indexes and performs the following actions (not necessarily in the specified order):

  • Runs ANALYZE TABLE to invalidate all plans for the table based on the statistics that are significantly different from the current statistics, but does not invalidate all other plans. However, ANALYZE TABLE requires a minimum threshold of changes in the table statistics to invalidate the existing plans for the table. Therefore, ANALYZE TABLE may not invalidate the plans unless the minimum threshold is reached. Hence, there is still a need to generate a new plan.

  • Runs the DROP FROM PLANCACHE command to ensure that plans are generated with the latest statistics and all the previously compiled query plans are invalidated and purged, both in-memory and on-disk. This command updates the stats and changes the indexes. It can be run either before or after ANALYZE TABLE.

The impact of recent index and stats changes are reflected in all the plans for all the queries, across the workspace.

Example 2: Testing first-run performance

The first time a query of a particular shape is run, it is compiled asynchronously for future invocations. This speeds up query execution time for long and complex queries, while at the same time providing efficient query plans for later use. See Code Generation for more information on query compilation. To test compilation and measure the first run performance accurately, run the DROP FROM PLANCACHE command on all nodes (aggregators and leaves) across the workspace.

Related Topics

Last modified: May 2, 2023

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