Join Indexes and the Optimizer
For each base table in a query, the Optimizer performs certain processing phases to decide how a database operation that uses a join index is to be processed.
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In this phase… |
The optimizer... |
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Qualification |
evaluates up to 10 join indexes to choose the one with the lowest cost. Qualification for the best plan includes one or more of the following benefits: |
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Analysis of results |
determines if this plan will result in unique results, analyzing only those tables in the query that are used in the join index. |
Subsequent action depends on analysis of the results.
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IF the results are... |
THEN the Optimizer... |
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unique |
skips the sort-delete steps used to remove duplicates. |
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nonunique |
determines whether eliminating all duplicates can still produce a valid plan, recognizing any case where the following things are true. |