查询优化器根据从每个表检索的行数估计查询成本。
The query optimizer bases query-cost estimates on the number of rows to be retrieved from each table.
生成访问计划，查看优化器对该查询返回的总基数(行数)的估计值。
Generate the access plan and see what the optimizer estimates the total cardinality (Number of rows) returned by the query will be.
此外，现在可以正确地估计结果集中的总行数。
Besides, the total number or rows in the result set is now estimated correctly.
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