Yes. In Microsoft SQL Server, a stored procedure that compiles successfully is not guaranteed to behave identically in every later execution. The database, compatibility level, session context, inputs, and compiled execution plan can change. A new or different plan may make the procedure slower without changing its logical results; a changed result or error requires checking the procedure’s actual execution context and data, not just its compile status.
What does successful compilation actually confirm?
A stored procedure has source code, an execution context, and a query plan. These are related, but they are not the same thing. Successful compilation confirms that SQL Server could compile the procedure or its statements in a particular database context. It does not certify that the database will remain unchanged or that every later call will use the same plan and conditions.
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SQL Server can keep a compiled plan in memory and reuse it while it remains available. Microsoft describes the engine detecting an existing plan and reusing it when a procedure runs again before that plan is aged out of memory. Reuse is conditional, not a promise that one plan will be used forever. Microsoft’s SQL Server query-processing guide explains plan reuse and recompilation.
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First identify what “differently” means: different rows or values, a new error, or slower execution. Those are distinct symptoms. SQL Server can recompile affected statements after changes to referenced tables or views, indexes, statistics, procedure definitions, and other execution conditions. Its query-processing guide also lists causes such as deferred compilation, changed SET options, and changes to temporary tables. A recompile means a statement is compiled again against the then-current context; by itself, it does not prove that the SQL logic changed.
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- Code change: Compare the current procedure definition with the version that produced the earlier behavior.
- Database change: Check changes to referenced objects, indexes, statistics, and data.
- Context change: Check the database compatibility level and relevant session SET options, as well as temporary-table definitions where applicable.
- Input change: Compare the parameter values and the order in which calls occurred.
Can a stored procedure compile but return different results?
It can, but a changed execution plan alone is not evidence that returned data changed. Plan choice primarily matters to performance; establish a semantic difference by comparing outputs or errors under controlled conditions and checking what changed between executions.
One SQL Server-specific behavior to examine is database compatibility level. Compatibility levels help manage upgrade risk from changes in query-optimization behavior. Microsoft documents an implicit conversion between datetime and datetime2 as an example of a compatibility-level-related behavior change. The exact effects depend on the SQL Server version, compatibility setting, expression, and data; do not generalize this example to other database engines. See Microsoft’s compatibility-level documentation.
Why did my stored procedure get slower after a database change?
A newly compiled plan can have different performance characteristics even when the procedure’s intended results are unchanged. Parameter sniffing is one reason: SQL Server can use parameter values available during compilation or recompilation when choosing a plan. If those values are unrepresentative of later calls, the chosen plan may perform poorly for those later inputs. That is a performance explanation, not proof of a logic or result change.
Plan reuse and recompilation involve a trade-off. Reuse avoids compiling the same statement repeatedly; recompilation lets SQL Server compile in a current context and can take current parameter values into account. Microsoft documents several recompilation choices, including recompiling a procedure, an individual statement, or on execution. Which approach is appropriate depends on the workload and the problem being addressed; recompilation is not a universal performance fix. Microsoft’s stored-procedure recompilation guide describes the options and parameter-sensitive behavior.
Does dynamic SQL always get a fresh plan?
No. With SQL Server’s sp_executesql, keeping the statement text constant and passing changing values as parameters can allow the plan to be reused. Different parameter values do not, by themselves, mean the statement was recompiled or that its meaning changed. Microsoft says the optimizer is likely to reuse a plan from an earlier execution when the statement text is the same and only parameter values vary. See the sp_executesql documentation.
How to investigate a result change or slowdown
- Reproduce the same call: Record the procedure definition, parameter values, database, and relevant session SET options for both the earlier and current cases.
- Record the environment: Identify the database engine and exact version, compatibility level, and changes to referenced schema, indexes, statistics, and data.
- Classify the symptom: Compare returned rows and values, errors, and elapsed performance separately. A slowdown is not the same as a result change.
- Inspect execution evidence: Compare available execution plans and use SQL Server’s Query Store and statement-recompilation diagnostics to investigate plan changes and recompilation. Microsoft’s query-processing guide covers recompilation reporting and Query Store plan behavior.
- Test a targeted change: If evidence points to parameter-sensitive performance, evaluate the appropriate recompilation option against the workload rather than adding
WITH RECOMPILEreflexively. A procedure recompile marks it for recompilation on its next execution; it does not run the procedure itself. See Microsoft’s recompilation guidance.
The sourced behaviors here describe Microsoft SQL Server. Other database engines have their own compilation, plan-cache, and compatibility rules, so verify the relevant product’s documentation before applying these details elsewhere.
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