What is SQL Server memory grant spill and how does it impact query execution?

Memory grant spill occurs when a query receives insufficient memory and spills intermediate data to tempdb.

Operators like sort and hash join require memory grants. If the granted memory is insufficient, SQL Server spills intermediate results to tempdb worktables. This introduces IO overhead and slows execution significantly. Spills often occur due to poor cardinality estimation or parameter sniffing. SQL Server may later adjust grants using memory grant feedback, but initial executions can still suffer.