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What is the difference between Estimated and Actual Execution Plans?

Expert MS SQL
Quick Answer Estimated plan uses statistics to guess row counts and show the planned approach. Actual plan captures what really happened — actual rows, actual executions, actual time. The gap between estimated and actual row counts is the first thing to look at. A big gap means bad statistics are misleading the optimizer.

Answer

The estimated plan shows SQL Server's predicted execution strategy based on statistics before running the query. The actual plan shows what really happened: row counts, spills, memory usage, and operator execution. Estimated plans are safe for production; actual plans reveal real bottlenecks like bad estimates, scans, or sorts. Both are essential for diagnosing performance issues.

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