Normally, the optimizer selects an execution plan for a query every time the query is executed. Optimizing at execution time allows the optimizer to choose a plan based on current system state, as well as the values of current selectivity estimates and estimates based on the values of host variables. For queries that are executed frequently, the cost of query optimization can outweigh the benefits of optimizing at execution time. To reduce the cost of optimizing these statements repeatedly, the optimizer caches plans for:
For INSERT statements, only INSERT...VALUES statements qualify for caching; INSERT...ON EXISTING statements do not qualify for caching.
For UPDATE and DELETE statements, the WHERE clause must be present and contain search conditions that uniquely identify a row using the primary key. No extra search conditions are allowed if plan caching is desired. Also, for UPDATE statements, a SET clause that contains a variable assignment disqualifies the statement from caching.
After one of these statements has been executed several times by a connection, the optimizer builds a reusable plan for the statement. A reusable plan does not use the values of host variables for selectivity estimation or rewrite optimizations. The reusable plan may have a higher cost because of this. If the reusable plan has the same structure as the plans observed in the previous executions of the statement, the database server chooses to add the reusable plan to a plan cache. Otherwise, the benefit of optimizing on each execution outweighs the savings from avoiding optimization, and the execution plan is not cached.
If an execution plan uses a materialized view that was not referenced by the statement, and the materialized_view_optimization option is set to something other than Stale, then the execution plan is not cached and the statement is optimized again the next time the stored procedure, user-defined function, or trigger is called.
The plan cache is a per-connection cache of the data structures used to execute an access plan. Reusing the cached plan involves looking up the plan in the cache and resetting it to an initial state. This is typically substantially faster than optimizing the statement. Cached plans may be stored to disk if they are used infrequently, and they do not increase the cache usage. The optimizer periodically re-optimizes queries to verify that the cached plan is still relatively efficient.
The maximum number of plans to cache is specified with the max_plans_cached option. The default is 20. To disable plan caching, set this option to 0. See max_plans_cached option [database].
You can use the QueryCachedPlans statistic to show how many query execution plans are currently cached. This property can be retrieved using the CONNECTION_PROPERTY function to show how many query execution plans are cached for a given connection, or the DB_PROPERTY function can be used to count the number of cached execution plans across all connections. This property can be used in combination with QueryCachePages, QueryOptimized, QueryBypassed, and QueryReused to help determine the best setting for the max_plans_cached option. See Connection-level properties.
You can use the database or QueryCachePages connection property to determine the number of pages used to cache execution plans. These pages occupy space in the temporary file, but are not necessarily resident in memory.
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