If I Execute The Same Query Changing the Parameter, Will The Execution Plan Change

11. December 2024 Mehedi SQL Server , SQL Tips Comments (0)

In a casual conversation with team, one of my junior team members asked, "If I execute the same query changing the parameter, will the execution plan change?" I thought for some moments and explained him. Here goes the synopsis of our discussion:

In SQL Server, if you run the same query changing the parameters, the execution plan change depends on several factors. For example: parameter sniffing, the Query Optimizer's behavior, and how database engine caches and reuses execution plans. Let's have a quick analysis.

Parameter Sniffing

  • What it is: When a query is executed, SQL Server "sniffs" the parameter values at compile time to generate an optimal execution plan.
  • Impact: If the first execution uses a parameter that is not representative of typical values (e.g., an outlier), the resulting execution plan might be sub-optimal for other parameter values. However, the same execution plan is reused for subsequent executions unless something triggers recompilation.

Diagram 1: - Parameter Sniffing

Plan Reuse

  • SQL Server caches execution plans to improve performance for repeated queries.
  • If you change the parameter but the structure of the query remains identical, SQL Server will often reuse the cached plan, even if it's not optimal for the new parameter.

Triggers for Plan Change

An execution plan might change due to:

  • Query or schema changes: Modifying the query text or changing indexes/statistics.
  • Recompilation triggers: Explicit recompilation (OPTION (RECOMPILE)), schema changes, or updates to statistics.
  • Cardinality issues: If parameter values lead to vastly different row estimates, SQL Server might generate a different plan.

Parameterized Queries vs. Ad-Hoc Queries

  • For parameterized queries, SQL Server typically generates a single execution plan for all parameter values.
  • For ad-hoc queries where the parameter is directly embedded in the query text, SQL Server might create separate plans for each distinct parameter value.

Using Query Hints

  • OPTION (RECOMPILE): Forces SQL Server to recompile the query for each execution, ensuring the plan is optimized for the specific parameter value.
  • OPTIMIZE FOR: Helps SQL Server optimize the plan for a specific parameter value.

Example of Plan Behavior

Suppose you have a query:

SELECT * 
FROM Sales 
WHERE Region = @Region;
  • Scenario 1: If @Region = 'North' returns many rows, SQL Server might generate a plan with a table scan.
  • Scenario 2: If @Region = 'South' returns only a few rows, an index seek might be better.
  • Without recompilation, the plan generated for 'North' might still be used for 'South', which could lead to performance issues.

Mitigation Strategies

  • Use Query Store to monitor and analyze execution plans for parameterized queries.
  • Force plan recompilation when needed, or consider query hints like OPTION (RECOMPILE) or OPTIMIZE FOR.
  • Use plan guides or stored procedures to handle parameter sensitivity more explicitly.

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