Exploiting Uniqueness in Query Optimization.

@inproceedings{DBLP:conf/icde/PaulleyL94,
  author    = {G. N. Paulley and
               Per-{\AA}ke Larson},
  title     = {Exploiting Uniqueness in Query Optimization},
  booktitle = {Proceedings of the Tenth International Conference on Data Engineering,
               February 14-18, 1994, Houston, Texas, USA},
  publisher = {IEEE Computer Society},
  year      = {1994},
  isbn      = {0-8186-5400-7},
  pages     = {68-79},
  ee        = {db/conf/icde/PaulleyL94.html},
  crossref  = {DBLP:conf/icde/94},
  bibsource = {DBLP, http://dblp.uni-trier.de}
}

Abstract

Consider an SQL query that specifies duplicate elimination via a DISTINCT clause. Because duplicate elimination often requires an expensive sort of the query result, it is often worthwhile to identify unnecessary DISTINCT clauses and avoid the sort altogether. We prove a necessary and sufficient condition for deciding if a query requires duplicate elimination. The condition exploits knowledge about keys, table constraints, and query predicates. Because the condition cannot always be tested efficiently, we offer a practical algorithm that tests a simpler, sufficient condition. We consider applications of this condition for various types of queries, and show that we can exploit this condition in both relational and non relational database systems.

Keywords: query optimization, DISTINCT, SQL, duplicate elimination

Copyright © 1994 by The Institute of Electrical and Electronic Engineers, Inc. (IEEE). Abstract used with permission.

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