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fix typo #11

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fix typo #11

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2 changes: 1 addition & 1 deletion zh-CN/4.sql-engine/9.choice-execution-plan.md
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Expand Up @@ -10,7 +10,7 @@

根据上节介绍的等价规则，连接操作是满足交换率的。假设当 n=5 时，可以在前三个的 12 种连接顺序中选择最好的一种与另外的两个进行连接，这样就又有 12 种连接顺序。对比原来的 5 个表进行任意连接导致较大的搜索空间而言，只有 12+12 种，这其实就是一种动态规划的思想。

另外这里面还涉及到 Interesting sort odrer 的概念，假设选择先进行前三个表的连接，采用的是 merge join（ 一般是有序的），即三个表连接之后会产生一个有序的顺序。这时需要判断这个有序的顺序能否在跟第四表和第五张表的连接中起到作用，如果能起到作用，那么这个连接就能产生一种叫做 Interesting sort odrer 的优势，通常也是会优先考虑这个优势的。
另外这里面还涉及到 Interesting sort order 的概念，假设选择先进行前三个表的连接，采用的是 merge join（ 一般是有序的），即三个表连接之后会产生一个有序的顺序。这时需要判断这个有序的顺序能否在跟第四表和第五张表的连接中起到作用，如果能起到作用，那么这个连接就能产生一种叫做 Interesting sort order 的优势，通常也是会优先考虑这个优势的。

## 计划树形状

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