Postgres效率

Question

让我们说我有一张桌子，里面有一些钥匙，帐号和点击。

哪个更快？我想我的问题是，如果postgres（或任何sql）有一个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;

OR

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;

我很想听到你要说的任何话，或者你可以提供的资源来理解这类事情。谢谢！

Answer 1

是的，确实如此。这就是函数波动的含义。

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

参见VOLATILE vs STABLE vs IMMUTABLE。