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Index Merge

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一条 SQL 是有可能用多个索引的

索引合并 - index merge

The Index Merge access method retrieves rows with multiple range scans and merges their results into one. This access method merges index scans from a single table only, not scans across multiple tables. The merge can produce unions, intersections, or unions-of-intersections of its underlying scans.

MySQL 5.1 引入, 适用于一张表内, 合并多个 range 索引扫描的结果, 来优化查询

EXPLAIN 输出

  • type: 显示 index_merge
  • key: 包含使用到的索引
  • key_len: 以列表形式展示每个合并索引中最长键部分的长度
  • Extra: 三种算法
    • Using intersect(...)
    • Using union(...)
    • Using sort_union(...)

See Also

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