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Abstract
En 中文
The outerjoin operator is currently available in the query language of several major DBMSs, and it is included in the proposed SQL2 standard draft. However, “associativity problems” of the operator have been pointed out since its introduction. In this paper we propose a shift in the intuition behind outerjoin: Instead of computing the join while also
preserving
its arguments, outerjoin delivers tuples that come
either
from the join
or
from the arguments. Queries with joins and outerjoins deliver tuples that come from one out of several joins, where a single relation is a trivial join. An advantage of this view is that, in contrast to preservation,
disjunction
is commutative and associative, which is a significant property for intuition, formalisms, and generation of execution plans.
Based on a disjunctive normal form, we show that some data merging queries cannot be evaluated by means of binary outerjoins, and give alternative procedures to evaluate those queries. We also explore several evaluation strategies for outerjoin queries, including the use of semijoin programs to reduce base relations.
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