MySQL查询优化概念辨析---Using where 和 Using index

MySQL的查询执行计划中，Extra列经常会出现“Using where; Using index”。

MySQL官方手册解释：

Using index
The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This strategy can be used when the query uses only columns that are part of a single index.
Using where
A WHERE clause is used to restrict which rows to match against the next table or send to the client. Unless you specifically intend to fetch or examine all rows from the table, you may have something wrong in your query if the Extra value is not Using where and the table join type is ALL or index.

这表明：

Using index不读数据文件，只从索引文件获取数据

Using where过滤元组和是否读取数据文件或索引文件没有关系

但MySQL手册在Using index处又说：

If the Extra column also says Using where, it means the index is being used to perform lookups of key values. Without Using where, the optimizer may be reading the index to avoid reading data rows but not using it for lookups. For example, if the index is a covering index for the query, the optimizer may scan it without using it for lookups.

所以有朋友如此理解：

1 没有Using where只有Using index：则不读数据文件，所有字段都可以从索引上获得（Without Using where, the optimizer may be reading the index to avoid reading data rows but not using it for lookups）

2 同时有Using where和Using index：因为“Without Using where...”这句上下文，则意味着同时有Using where和Using index则一定需要读取数据文件

其实不然。“Using where只是过滤元组，和是否读取数据文件或索引文件没有关系”。

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