ASA SQL User's Guide
Query Optimization and Execution
Semantic query transformations
The optimizer can perform a number of transformations in search of more efficient and convenient representations of your query. Because the optimizer performs these transformations, the plan may look quite different from a literal interpretation of your original query. Common manipulations include:
unnecessary DISTINCT elimination
subquery unnesting
predicate pushdown in UNION or GROUPed views
join elimination
optimization for minimum or maximum functions
OR, in-list optimization
LIKE optimizations
conversion of outer joins to inner joins
discovery of exploitable conditions through predicate inference
elimination of unnecessary case translation
The following subsections discuss each of these operations.
Unnecessary DISTINCT elimination
Subquery unnesting
Predicate pushdown into GROUPed or UNION views
Join elimination
Optimization for minimum or maximum functions
IN-list optimization
LIKE optimizations
Conversion of outer joins to inner joins
Discovery of exploitable conditions
Elimination of unnecessary case translation