Question

根据我对多列索引的理解，它们仅在您从左侧开始使用列而不跳过任何列时才有用。例如，如果您有(a, b, c)的索引，则可以在a，a, b或a, b, c上进行查询。

但今天我发现当在InnoDB表上有一个索引（BTREE）时：

some_varchar, some_bigint, other_varchar

我可以查询：

SELECT MAX(some_bigint) FROM the_table

并且它的计划说：

           id: 1
  select_type: SIMPLE
        table: the_table
         type: index
possible_keys: NULL
          key: index_some_varchar_some_bigint_other_varchar
      key_len: 175
          ref: NULL
         rows: 1
        Extra: Using index

这似乎不同意the docs。自key设置后，它也会造成混淆，但possible_keys不是。

这在实践中如何运作？如果密钥首先由some_varchar排序，（或者它的前缀），MySQL如何从中获取第二列的MAX？

（猜测是MySQL会收集一些关于索引中所有列的额外信息，但如果这是真的 - 是否可以直接看到它？）

Answer 1

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Using index并不一定意味着通过快速查找访问了该值。它只是意味着没有访问行数据。如果Using index为type且index为Extra，则仍表示正在扫描整个索引：

来自the documentation：

索引连接类型与ALL相同，但扫描索引树除外。

对于实际使用索引前缀的Using index查找，解释如下：

MAX

为什么MAX在索引中间的列上工作

1 个答案: