Postgres efficiency

Lets say that I have a table with some key, accounts, and hits.

Which is faster? I guess my question is if postgres's (or any sql for that matter) has an optimizer intelligent to see identical functions in a query

SELECT key, accounts, hits,1.0*hits/accounts as ratio FROM
(
 开发者_开发技巧   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;

I'd love to hear anything you have to say or resources you can provide on understanding this sort of thing. Thanks!

---

Yes, it is. That is what function volatility is for.

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

See VOLATILE vs STABLE vs IMMUTABLE.