Plan Caching and Reuse
When you submit a SQL query, the engine parses, analyzes, and optimizes it before generating an execution plan. This plan describes the steps and operators the engine will use to retrieve your results. Creating a plan can be computationally expensive, especially for complex queries. To improve efficiency, SQL engines often use plan caching: after a plan is generated for a query, it is stored in memory. If the same or a sufficiently similar query is executed again, the engine can reuse the cached plan instead of creating a new one from scratch. This approach is especially effective for queries that are run repeatedly, such as those in applications or reporting tools.
Plan caching typically works best when queries are identical in structure. Some engines can also recognize and reuse plans for parameterized queries, where only the input values change but the query shape remains the same. The cache is usually managed automatically, with the engine deciding how many plans to keep and when to evict old or unused entries.
12345-- Running the same SELECT statement multiple times SELECT name, salary FROM employees WHERE department_id = 2; -- Running again (the engine can reuse the cached plan) SELECT name, salary FROM employees WHERE department_id = 2;
In these two queries, the SQL engine can detect that the query structure is identical. After the first execution, the engine stores the plan in the cache. When you run the second query, the engine quickly finds the existing plan and executes it, skipping the full planning process. This reuse saves time and resources, especially in systems that serve many repeated queries.
Plan caching is most beneficial for queries that are executed frequently and do not change in structure. Applications that use parameterized queries—where only the values supplied change—can see significant performance improvements. However, plan caching may not be used if queries differ in structure, reference different objects, or if the engine detects that a cached plan is no longer valid due to schema changes or updated statistics. Some engines may also bypass the cache for ad-hoc queries or when hints or session settings change.
12345678-- Example of a parameterized query that benefits from plan caching -- The query structure is the same, but the parameter changes -- First execution with department_id = 3 SELECT name, salary FROM employees WHERE department_id = 3; -- Second execution with department_id = 4 SELECT name, salary FROM employees WHERE department_id = 4;
1. What is the main advantage of plan caching in SQL engines?
2. Under what circumstances might a cached plan NOT be reused?
3. Fill in the blank: Plan caching helps reduce the ________ of query planning for repeated statements.
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When you submit a SQL query, the engine parses, analyzes, and optimizes it before generating an execution plan. This plan describes the steps and operators the engine will use to retrieve your results. Creating a plan can be computationally expensive, especially for complex queries. To improve efficiency, SQL engines often use plan caching: after a plan is generated for a query, it is stored in memory. If the same or a sufficiently similar query is executed again, the engine can reuse the cached plan instead of creating a new one from scratch. This approach is especially effective for queries that are run repeatedly, such as those in applications or reporting tools.
Plan caching typically works best when queries are identical in structure. Some engines can also recognize and reuse plans for parameterized queries, where only the input values change but the query shape remains the same. The cache is usually managed automatically, with the engine deciding how many plans to keep and when to evict old or unused entries.
12345-- Running the same SELECT statement multiple times SELECT name, salary FROM employees WHERE department_id = 2; -- Running again (the engine can reuse the cached plan) SELECT name, salary FROM employees WHERE department_id = 2;
In these two queries, the SQL engine can detect that the query structure is identical. After the first execution, the engine stores the plan in the cache. When you run the second query, the engine quickly finds the existing plan and executes it, skipping the full planning process. This reuse saves time and resources, especially in systems that serve many repeated queries.
Plan caching is most beneficial for queries that are executed frequently and do not change in structure. Applications that use parameterized queries—where only the values supplied change—can see significant performance improvements. However, plan caching may not be used if queries differ in structure, reference different objects, or if the engine detects that a cached plan is no longer valid due to schema changes or updated statistics. Some engines may also bypass the cache for ad-hoc queries or when hints or session settings change.
12345678-- Example of a parameterized query that benefits from plan caching -- The query structure is the same, but the parameter changes -- First execution with department_id = 3 SELECT name, salary FROM employees WHERE department_id = 3; -- Second execution with department_id = 4 SELECT name, salary FROM employees WHERE department_id = 4;
1. What is the main advantage of plan caching in SQL engines?
2. Under what circumstances might a cached plan NOT be reused?
3. Fill in the blank: Plan caching helps reduce the ________ of query planning for repeated statements.
Thanks for your feedback!
