| 摘要 |
A query optimizer determines the applicability of materialized views to a query. View utilization alternatives are generated in the exploration stage of optimization, so that interaction with other transformations in complex queries is taken into account. A final decision on whether to use a materialized view is based on estimated cost. The optimizer generates a table of alternatives, which compactly encodes the various possibilities for each sub-expression of the query. Optimal-cost operator trees are extracted from this table. Materialized views are detected and substituted during exploration of the various possibilities and added to the table of alternatives. Materialized views and the alternatives are selected for use in a query execution plan based on cost. When two operator trees are not identical, a residual operator can be used if one operator tree subsumes the other operator tree. The residual expression can contain operators such as filters, group by and join.
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