In the Query Rewrite phase, SQL Anywhere performs semantic transformations in search of more efficient and convenient representations of the query. Because the query may be rewritten into a semantically equivalent query, the plan may look quite different from your original query. Common manipulations include:
eliminating unnecessary DISTINCT conditions
performing a predicate push-down in UNION or GROUPed views and derived tables
optimizing of OR and IN-list predicates
optimizing of LIKE predicates
converting outer joins to inner joins
eliminating outer joins and inner joins
discovering exploitable conditions through predicate inference
eliminating unnecessary case translation
rewriting subqueries as EXISTS predicates
inferring sargable IN predicates, which can be used for partial index scans from OR predicates that cannot be transformed into AND predicates
Some query rewrite optimizations cannot be performed on the main query block if the cursor is updatable. Declare the cursor as read-only to take advantage of the optimizations.
Some of the rewrite optimizations performed during the Query Rewrite phase can be observed in the results returned by the REWRITE function.
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