Luckily, cardinality estimation is a popular area of research.
幸运的是，基数估计是一个热门的研究领域。
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。
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