Aiming at how the DBMS query optimizer to generate the smallest cost query plan, optimization rules of the relational algebra expression are given.

针对DBMS查询优化器如何生成成本最小的查询计划问题，给出关系代数表达式的优化规则。

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Is also a great introduction to this topic and shows the motivation and theory behind the "relational algebra" in ActiveRecord 3.

也是一个关于此话题的很好的介绍，解释了ActiveRecord 3中的“关系代数”背后的的动机和理论。

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This theoretical model allows, for example, that relational algebra operations to be replaced by more efficient equivalent operations whereas not interfere with the result.

例如，这种理论模型允许用更有效的等价操作来代替关系代数操作同时不影响结果。

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