| From: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Gregory Stark <stark(at)enterprisedb(dot)com>, "Alexey A(dot) Nalbat" <nalbat(at)price(dot)ru>, Decibel! <decibel(at)decibel(dot)org>, pgsql-general(at)postgresql(dot)org |
| Subject: | Re: unnesesary sorting after Merge Full Join |
| Date: | 2008-02-26 15:49:10 |
| Message-ID: | 1204040950.4252.350.camel@ebony.site |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-general |
On Tue, 2008-02-26 at 10:34 -0500, Tom Lane wrote:
> > and add COALESCE(id2,id1) to it as well I think.
>
> No, because those two expressions are not equivalent. (Hmm ... squint
> ... but full merge join is pretty much symmetric, so it's not clear
> why it should matter which side is left or right. Maybe COALESCE
> isn't
> exactly the right concept with which to describe the merged variable?)
It is, in this case only, since when id2 is not null id2 == id1.
So in this case its OK to express a symmetric relationship as a left
handed function.
--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Yuri Gordienko | 2008-02-26 15:50:32 | Custom conversion |
| Previous Message | Maciej Sieczka | 2008-02-26 15:47:04 | Re: how to auto GRANT custom ACL on a new table? |