Postgres效率

Question

假设我有一个表，其中包含一些键、帐户和点击数。

哪个更快？我想我的问题是postgres(或任何sql )是否有智能优化器来查看查询中相同的函数

SELECT key, accounts, hits,1.0*hits/accounts as ratio FROM
(
    SELECT key, COUNT(DISTINCT accounts) as accounts, SUM(hits) as hits
    FROM table
    GROUP BY key
) a;

或

SELECT key, COUNT(DISTINCT accounts) as accounts, SUM(hits) as hits, 1.0*SUM(hits)/COUNT(DISTINCT accounts) as ratio
FROM table
GROUP BY key;

我很乐意听到你有什么要说的，或者你能提供的关于理解这类事情的资源。谢谢!

Answer 1

是的，是这样的。这就是函数波动性的用途。

http://www.postgresql.org/docs/current/static/sql-createfunction.html

参见VOLATILE vs稳定vs IMMUTABLE。