| 摘要 |
A query optimization technique that determines whether any column from a right quantifier of a left outer join query is projected out of the query. If not, then all predicates in an ON clause are removed from the left outer join query, the right quantifier is removed from the left outer join query, and the left outer join query is converted to a simple select query. A number of steps are performed to determine whether any of the columns quantified by the right quantifier participate in a projection list of the query. A set of equivalence class columns is computed for the query, wherein the equivalence classes are derived from equi-join predicates in the query. A set of columns quantified by the right quantifier across the computed set of equivalence classes is computed. A determination is made whether a subset of the set of columns form a key for the right quantifier. If the right quantifier columns are not projected out of the query, then the optimization may be performed.
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