postgresql COUNT(DISTINCT ...) 很慢 [英] postgresql COUNT(DISTINCT ...) very slow

问题描述

我有一个非常简单的 SQL 查询:

I have a very simple SQL query:

SELECT COUNT(DISTINCT x) FROM table;

我的表有大约 150 万行.这个查询运行得很慢；大约需要 7.5 秒，相比之下

My table has about 1.5 million rows. This query is running pretty slowly; it takes about 7.5s, compared to

 SELECT COUNT(x) FROM table;

大约需要 435 毫秒.有什么方法可以更改我的查询以提高性能吗?我尝试过分组并进行常规计数，以及在 x 上放置索引；两者具有相同的 7.5 秒执行时间.

which takes about 435ms. Is there any way to change my query to improve performance? I've tried grouping and doing a regular count, as well as putting an index on x; both have the same 7.5s execution time.

推荐答案

你可以使用这个:

SELECT COUNT(*) FROM (SELECT DISTINCT column_name FROM table_name) AS temp;

这比:

COUNT(DISTINCT column_name)

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