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Why do row estimation errors cause performance issues?

Expert MS SQL
Quick Answer Row estimation errors cause the optimizer to pick wrong strategies. If it thinks a join produces 100 rows but it produces 1 million, it might choose Nested Loops (correct for 100 rows) which is catastrophic at 1 million. Estimation errors cascade — each wrong estimate compounds into a progressively worse plan.

Answer

SQL Server uses estimated row counts to choose join types, memory grants, and index strategies. If estimates are inaccurate, SQL Server may choose poor plans. Overestimation causes excessive memory allocation, while underestimation causes spills, nested loops, and excessive lookups. Accurate cardinality estimation is fundamental to stable performance.

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