Why this problem matters
When different data types are compared, SQL Server may apply an implicit conversion at runtime. If conversion occurs on the indexed column side, index seeks and cardinality estimates can be impaired.
How to diagnose it
Search cached plans for CONVERT_IMPLICIT and compare actual query parameter types with the schema. Not every implicit conversion is harmful; verify operator location and actual read impact.
The query lists only cached plans containing CONVERT_IMPLICIT. The cache may be incomplete, and a match alone does not prove that the conversion causes a performance problem.
SELECT TOP (50)
qs.execution_count,
qs.total_worker_time,
qs.total_logical_reads,
st.text AS batch_text,
qp.query_plan
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS st
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) AS qp
WHERE qp.query_plan.exist(
'declare default element namespace "http://schemas.microsoft.com/sqlserver/2004/07/showplan";
//ScalarOperator[contains(@ScalarString,"CONVERT_IMPLICIT")]') = 1
ORDER BY qs.total_worker_time DESC;A safe solution approach
Align application parameters and query expressions with the target column type. Correcting the data model and boundary-layer types is generally safer than converting the column in a predicate.
Why continuous monitoring matters
New application releases or schema changes can introduce conversions that did not exist before. moon can track duration and read trends and surface regressions through Slack, PagerDuty, or webhooks.
Frequently asked questions
Is every CONVERT_IMPLICIT a performance defect? Will an explicit CAST always fix it?
No, some conversions on constants or small data sets are insignificant. An explicit conversion helps only when applied on the correct side with a suitable type, and plan impact must be measured.
How does moon help with this problem?
moon continuously observes SQL Server, PostgreSQL, and MongoDB signals, helping teams evaluate the problem as a trend instead of relying on a one-time check.