The Eager Spool logical operator will consume the entire input, storing each row in a hidden temporary object stored in the tempdb database. If the operator is rewound (for example, by a Nested Loops operator) but no rebinding is needed, the spooled data is used instead of rescanning the input. If rebinding is needed, the spooled data is discarded and the spool object is rebuilt by rescanning the (rebound) input.
The Eager Spool operator will build its spool file eagerly. When the spool’s parent operator asks for the first row, the spool operator will consume all rows from its input operator and store them in the spool.
Note An alternative way of building a spool file is with the Lazy Spool operator.