The Eager Spool operator takes its entire input into a temporary object
The Eager Spool operator builds a hidden temporary object in tempdb and stores the entire input. The 'eager' part means as soon the the parent requests the first row, Eager Spool consumes the whole input in one go.
- Microsoft Documentation - Showplan Logical and Physical Operators Reference
- Microsoft Documentation - Explain Plans
- Plansplaining - Hugo Kornelis' Blog series
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