Efficient Exploitation of Similar Subexpressions for Query Processing
Jingren Zhou, Paul Larson, Johann-Christoph Freytag, Wolfgang Lehner

Abstract. Complex queries often contain common or similar
subex-pressions, either within a single query or among multiple
queries submitted as a batch. If so, query execution time can be
improved by evaluating a common subexpression once and reusing the
result in multiple places. However, current query optimizers do not
recognize and exploit similar subex-pressions, even within the same
query.

In this paper, we present an efficient, scalable, and principled
solution to this long-standing optimization problem. We introduce a
light-weight and effective mechanism to detect potential sharing
opportunities among expressions. Candidate shared subexpressions are
constructed and optimization is resumed to determine which, if any,
such subexpressions to include in the final query plan. The chosen
subexpression(s) are computed only once and the results are reused to
answer other parts of queries. Our solution automatically applies to
optimization of query batches, nested queries, and maintenance of
multiple materialized views. Experiments show signifcant performance
improvements with minimal overhead.