PostgreSQL性能相关的必知SQL

执行计划

option可以为ANALYZE/VERBOSE/COSTS/BUFFERS/FORMAT

理解cost

EXPLAIN输出结果中的cost，其中有两个值，中间用..隔开，第一个数字是启动成本，即返回第一行需要的成本，第二个数字表示返回所有数据的成本。而程本cost描述了执行一个SQL的代价是多少，默认情况下不同的操作所带来不同的cost值：

• 顺序扫描一个数据块，cost为1
• 随机扫描一个数据块，cost为4
• 处理一个数据行的cpu，cost为0.01
• 处理一个索引列的cpu，cost为0.005
• 每个操作符的cpu，cost为0.0025

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