What do you mean by evaluation expression?
EVALUATION OF EXPRESSION
We have studied how individual relational operations are carried out. The obvious way to evaluate an expression is simply to evaluate one operation at a time, in an appropriate order. Now we consider how to evaluate an expression containing multiple operations. There are two approaches to how a query execution tree can be evaluated:
Materialization: Compute the result of an evaluation primitive and materialize (store) the new relation on the disk.
Pipelining: Pass on tuples to parent operations even while an operation is still being executed.
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