Why SQL Queries Run Fast in Testing but Time Out in Production
A common but frustrating database issue called parameter sniffing causes SQL Server to compile an execution plan based on the first set of parameter values it encounters, then reuse that plan for all subsequent queries. When later queries involve vastly different data volumes, the cached plan becomes inefficient — for example, an Index Seek plan built for 5 rows performs poorly when applied to 500,000 rows. Developers can diagnose the problem by querying SQL Server's plan cache using Dynamic Management Views (DMVs) to compare the parameter values used at compile time versus runtime. If those values differ significantly, parameter sniffing is likely the cause of the slowdown. Common fixes include using OPTIMIZE FOR UNKNOWN query hints, relying on local variables in stored procedures, refreshing stale index statistics, or leveraging SQL Server's Query Store to enforce a known good execution plan.
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