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Abstract

A method and apparatus for optimizing SQL queries in a relational database management system uses early-out join transformations. An early-out join comprises a many-to-one existential join, wherein the join scans an inner table for a match for each row of the outer table and terminates the scan for each row of the outer table when a single match is found in the inner table. To transform a many-to-many join to an early-out join, the query must include a requirement for distinctiveness, either explicitly or implicitly, in one or more result columns for the join operation. Distinctiveness can be specified using the DISTINCT keyword in the SELECT clause or can be implied from the predicates present in the query. The early-out join transformation also requires that no columns of the inner table be referenced after the join, or if an inner table column is referenced after the join, that each referenced column be 'bound'. A referenced column can be bound in one of three ways: (1) an inner table column can be bound to a constant through an equality predicate, (2) an inner table column can be bound to an outer table column, or (3) an inner table column can be bound to a correlated value, wherein the correlated value originates outside the query block. In all three cases, an inner table column can be bound through the transitivity of equality predicates.

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