Optimized query plans are partitioned into equivalence groups that each include equivalence classes. Each equivalence group corresponds to a particular compiled, normalized, and parameterized query plan prior to optimization. Each equivalence class within an equivalence group corresponds to a different query plan corresponding to the particular compiled, normalized, and parameterized query plan represented by the equivalence group that has been optimized with respect to the selectivity of one or more predicate clauses of the query that is compiled to produce the particular compiled, normalized, and parameterized query plan. Optimized query plans are cached according to their respective equivalence groups and equivalence classes. When a query, similar to a query already compiled, optimized, and cached, is subsequently received and compiled, a selectivity for a predicate of the compiled query is computed, allowing the database management system to retrieve a cached query plan optimized for a similar query with similar selectivity.
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