Evaluation of Expressions

In our previous sections, we understood various concepts in query processing. We learned about the query processing steps, selection operations, and also several types of algorithms used for performing the join operation with their cost estimations.

We are already aware of computing and representing the individual relational operations for the given user query or expression. Here, we will get to know how to compute and evaluate an expression with multiple operations.

For evaluating an expression that carries multiple operations in it, we can perform the computation of each operation one by one. However, in the query processing system, we use two methods for evaluating an expression carrying multiple operations. These methods are:

  1. Materialization
  2. Pipelining

Let's take a brief discussion of these methods.


In this method, the given expression evaluates one relational operation at a time. Also, each operation is evaluated in an appropriate sequence or order. After evaluating all the operations, the outputs are materialized in a temporary relation for their subsequent uses. It leads the materialization method to a disadvantage. The disadvantage is that it needs to construct those temporary relations for materializing the results of the evaluated operations, respectively. These temporary relations are written on the disks unless they are small in size.


Pipelining is an alternate method or approach to the materialization method. In pipelining, it enables us to evaluate each relational operation of the expression simultaneously in a pipeline. In this approach, after evaluating one operation, its output is passed on to the next operation, and the chain continues till all the relational operations are evaluated thoroughly. Thus, there is no requirement of storing a temporary relation in pipelining. Such an advantage of pipelining makes it a better approach as compared to the approach used in the materialization method. Even the costs of both approaches can have subsequent differences in-between. But, both approaches perform the best role in different cases. Thus, both ways are feasible at their place.

We have described and discussed the materialization as well as pipelining method deeply in our next sections one by one.

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Evaluation of Expressions