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Yes. A SQL Server stored procedure can compile successfully and later behave differently because compilation is tied to a database and execution context, not a guarantee that those conditions will stay unchanged. A changed result or error is different from a slower execution plan: a new plan can affect performance without proving that the procedure’s logic or returned data changed.
What successful compilation does—and does not—confirm
A stored procedure has source code, an execution context, and a query plan. These are related, but they are not interchangeable. In SQL Server, the engine compiles procedure statements into an execution plan and can reuse a cached plan while it remains available. Microsoft’s Query Processing Architecture Guide explains that SQL Server detects an existing plan and reuses it when the procedure runs again before that plan ages out of memory.
A successful compile confirms that the procedure could be compiled in a particular context. It does not guarantee that future calls will use the same plan, parameter values, database state, compatibility behavior, or session settings.
Why does my stored procedure work differently now?
The database or execution context changed
Changes to a procedure, referenced tables or views, indexes, statistics, and other execution conditions can invalidate a plan or cause SQL Server to recompile affected statements. Microsoft also lists changes involving SET options and temporary tables among statement-recompilation causes. The newly compiled statement is evaluated against the database state and context in effect at that time.
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Database compatibility level is another possible source of behavioral differences. Microsoft documents compatibility levels as a way to limit upgrade risk from query-optimization behavior changes, and gives an implicit conversion between datetime and datetime2 as an example of a behavior that can differ. See Microsoft’s ALTER DATABASE compatibility-level documentation. This is a SQL Server-specific example, not a rule that applies identically to every database engine.
Compilation values influenced the plan
SQL Server can use parameter values available during compilation or recompilation to choose a plan. This behavior, commonly called parameter sniffing, can produce a plan that works well for one set of inputs but poorly for later inputs with a different data distribution. That can explain why a procedure got slower without showing that its logical results changed.
A dynamic statement reused its plan
Dynamic SQL is not necessarily compiled from scratch for every call. When sp_executesql receives the same statement text with different parameter values, SQL Server is likely to reuse a plan from an earlier execution. Parameter values changing from call to call do not by themselves establish that the statement’s meaning changed. See Microsoft’s sp_executesql documentation.
Can a stored procedure compile but return different results?
It can, but compilation alone does not establish why. A changed result or error calls for checking whether the procedure definition, database version or compatibility level, schema, relevant data, inputs, or session settings changed. A performance regression is a separate observation: plan selection and recompilation can change execution speed without demonstrating a change in logical results.
- Different rows or values: compare the before-and-after outputs using the same inputs and relevant database state.
- A different error: record the exact error and check for changes to conversions, schema, compatibility settings, and session context.
- Slower execution only: investigate the plan, parameter values, statistics, and data distribution before treating it as a correctness problem.
How to diagnose the change
Start with a reproducible before-and-after case. Keep the observations separate: what the procedure returned, whether it raised an error, and how long it took are different questions.
- Identify the engine and version. The mechanisms described here are specific to SQL Server; do not assume another database engine follows the same rules.
- Compare the procedure definition and database settings. Record the procedure text, database version, and compatibility level for each case.
- Check database objects and statistics. Look for changes to referenced tables or views, indexes, statistics, and temporary-table shape.
- Record inputs and execution context. Capture representative parameter values, their order of execution, relevant session SET options, and the data distribution involved.
- Compare the outcome and plan evidence. Save the outputs or errors and inspect execution-plan changes. SQL Server’s Query Store and statement-recompilation diagnostics can help investigate plan changes, as described in the query-processing guide.
Should you recompile the procedure?
Not reflexively. Recompilation can be useful for parameter-sensitive performance problems, but the appropriate option depends on the workload and the scope of the issue. Microsoft’s stored-procedure recompilation guidance describes several options. Recompiling a procedure marks it for recompilation on its next execution; the recompile operation itself does not execute the procedure. Recompilation is a plan-management choice, not proof that the procedure’s results were wrong.
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