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。
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。
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