Indexes on the foreign keys referencing tables with few rows have low selectivity, and the utility of requiring the index has always been questionable.

外键上的索引所引用的表行数较少，选择性较少，需要索引的实用程序一直遭受质疑。

youdao

Selectivity: When the query optimizer (QO) studies your SQL, the degree of selectivity determines if an index should be used to perform the operation.

选择性：当查询优化器(query optimizer，即QO)读取SQL时，选择性的程度决定了索引是否应该用来执行该操作。

youdao

There is much less index access and no index AND'ing so that the overall performance can be one or two order of magnitude better - depending on predicate selectivity.

只有相当少的索引访问且没有索引 AND'ing，这样整体性能将提高一个或两个数量级 —— 取决于谓词选择。

youdao