Data Monitoring / SQL Server Query Performance
SQL Server: Implicit conversion predicates
A filtered lookup becomes a scan after an application type change. Compare parameter types, column types and execution-plan conversion warnings.
Environment and inputs: SQL Server 2019–2025 where the referenced feature is supported; Query Store and diagnostic collection require appropriate configuration and permissions. Azure SQL behavior can differ.
Failure scenario
A filtered lookup becomes a scan after an application type change. Confirm the cause before choosing a fix.
Evidence to collect
Compare parameter types, column types and execution-plan conversion warnings. Record the time, affected scope and a healthy comparison interval.
Fix the confirmed cause
Align parameter and column types and verify that result semantics remain unchanged. Write the expected result first. Change one relevant setting at a time.
Check the result and hand over
Repeat the check for implicit conversion predicates using the same scope and a representative interval. Check that the symptom is resolved, expected records are present and downstream results are correct. If coverage, correctness or latency worsens, restore the recorded configuration through the agreed rollback process.
Scope and precautions
Prefer read-only access and limit queries to the relevant period or dataset. This example has not been tested on your systems. Test changes in an approved environment. Keep credentials and sensitive data out of shared evidence.