Use of Algebraic Properties For Query Optimization
Queries can be represented as a tree, where
- the internal nodes are operators,
- leaves are relations,
- subtrees are subexpressions.
Our primary goal is to transform expression trees into equivalent expression trees, where the average size of the relations yielded by subexpressions in the tree are smaller than they were before the optimization. Our secondary goal is to try to form common subexpressions within a single query, or if there are more than one queries being evaluated at the same time, in all of those queries. The rationale behind that second goal is that it is enough to compute common subexpressions once, and the results can be used in all queries that contain that subexpression.
Here we present a set of rules, that can be used in such transformations.
Read more about this topic: Relational Algebra
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