NDLI: Predicate migration:optimizing queries with expensive predicates

Content Provider	ACM Digital Library
Author	Stonebraker, Michael Hellerstein, Joseph M.
Abstract	The traditional focus of relational query optimization schemes has been on the choice of join methods and join orders. Restrictions have typically been handled in query optimizers by “predicate pushdown” rules, which apply restrictions in some random order before as many joins as possible. These rules work under the assumption that restriction is essentially a zero-time operation. However, today's extensible and object-oriented database systems allow users to define time-consuming functions, which may be used in a query's restriction and join predicates. Furthermore, SQL has long supported subquery predicates, which may be arbitrarily time-consuming to check. Thus restrictions should not be considered zero-time operations, and the model of query optimization must be enhanced.In this paper we develop a theory for moving expensive predicates in a query plan so that the total cost of the plan — including the costs of both joins and restrictions — is minimal. We present an algorithm to implement the theory, as well as results of our implementation in POSTGRES. Our experience with the newly enhanced POSTGRES query optimizer demonstrates that correctly optimizing queries with expensive predicates often produces plans that are orders of magnitude faster than plans generated by a traditional query optimizer. The additional complexity of considering expensive predicates during optimization is found to be manageably small.
Starting Page	267
Ending Page	276
Page Count	10
File Format	PDF
ISSN	01635808
DOI	10.1145/170036.170078
Journal	ACM SIGMOD Record (SGMD)
Volume Number	22
Issue Number	2
Language	English
Publisher	Association for Computing Machinery (ACM)
Publisher Date	1999-06-01
Publisher Place	New York
Access Restriction	One Nation One Subscription (ONOS)
Content Type	Text
Resource Type	Article
Subject	Information Systems Software

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