摘要： 1、by keeping them "independent" and then aggregating their aggregates -- we significantly decreased the amount of processing going on.在写统计个数的SQL语句的过程，我们最好，先把每个union all的子集的结果进行分别的汇总，然后，最终统一进行汇总，例如下面两个语句的写法：A:SELECT COUNT(*) FROM (SELECT NULL FROM test_explan UNION ALL SELECT ... 阅读全文