Up: Query Processing
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- These are only some of the many query-processing strategies used in
- Most systems only implement a few strategies.
- Some systems make a heuristic guess of a good strategy,
in order to minimize the number of strategies to be considered.
- Then the optimizer considers every possible strategy, but quits as
soon as it determines that the cost is greater than the best previously
- To simplify the strategy selection task, a query may be split into
- This simplifies strategy selection and permits recognition of common
sub-queries (no need to compute them twice).
- Examination of a query for common subqueries and the estimation of the
cost of a large number of strategies impose a substantial overhead on query
- However, this is usually more than offset by savings at query execution
- Therefore, most commercial systems include relatively sophisticated
Sun Jul 26 17:45:14 PDT 1998