To better illustrate the improvement in cardinality estimation, drop all the existing indexes on the EMPLOYEE table except the primary key index.
为了更好地解释说明基数估计值的提高，撤销除主键索引外的所有employee表上的现有索引。
Cardinality estimation is a process by which the optimizer USES statistics to determine the size of partial query results after predicates are applied or aggregation is performed.
基数估计是这样一种过程：在应用了谓词或执行了聚集之后，优化器使用统计信息确定部分查询结果的大小。
Cardinality estimation is a process by which the optimizer USES statistics to determine the size of partial query results after predicates are applied or aggregation is performed.
基数估计是这样一种过程：在应用了谓词或执行了聚集之后，优化器使用统计信息确定部分查询结果的大小。
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