| From: | Manfred Koizar <mkoi-pg(at)aon(dot)at> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Jean-Luc Lachance <jllachan(at)sympatico(dot)ca>, Mark Cave-Ayland <m(dot)cave-ayland(at)webbased(dot)co(dot)uk>, pgsql-general(at)postgresql(dot)org |
| Subject: | Re: Column as result of subtraction of two other columns? |
| Date: | 2004-08-23 16:59:04 |
| Message-ID: | 288ki01a8oo4598h7effii1p6iqn244sq6@email.aon.at |
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| Lists: | pgsql-general |
On Fri, 16 Jul 2004 12:04:54 -0400, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> select sum(x), sum(y), sum(x)-sum(y) from ...
>
>At least since 7.4, the system will notice the duplicate aggregates
>and run only two summations to compute the above, followed by a single
>subtraction at the end. The apparently more intelligent way suggested
>by Jean will have to run three summations, and thus end up being a net
>loss.
Also note that Jean-Luc's
select sum( x), sum( y), sum(x-y) from whatever group by z;
gives a different result in the presence of NULLs.
Servus
Manfred
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