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What happens when a query spills to TempDB?

Expert MS SQL
Quick Answer When a sort or hash operation needs more memory than its grant, SQL Server writes intermediate data to TempDB. This turns an in-memory operation into disk I/O — often 10-100x slower. You'll see Sort Warnings or Hash Warnings in the execution plan. Fix: update statistics, or use OPTION(MIN_GRANT_PERCENT) hints.

Answer

A query spills when SQL Server lacks sufficient memory for operations like sort or hash. SQL offloads intermediate results to TempDB, causing heavy disk I/O and slow execution. Frequent spills indicate bad estimates, missing indexes, or insufficient memory. Repeated spills degrade overall SQL Server performance.

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