What do you mean by evaluation expression?

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 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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