这个参数设置用于 map 端排序的缓冲区大小，单位是 MB，默认值是 100。

This parameter sets the buffer size for map-side sorting, in units of MB, 100 by default.

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当 MySQL 必须要进行排序时，就会在从磁盘上读取数据时分配一个排序缓冲区来存放这些数据行。

When MySQL must perform a sort, it allocates a sort buffer to store the rows as they're read from disk.

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—一个选择和排序组件，可以在可用区与选定区之间移动项目，还可以在选定区中上下移动项目。

A selection and ordering component that can move items between available and selected areas, and up and down in the selected area. Figure 2 shows an example.

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