| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Amit Kapila <amit(dot)kapila(at)huawei(dot)com> |
| Cc: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, "'Kyotaro HORIGUCHI'" <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Identity projection |
| Date: | 2013-03-14 15:04:51 |
| Message-ID: | 10977.1363273491@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
I'm starting to review this patch now, and I'm having a hard time with
this particular design decision:
Amit Kapila <amit(dot)kapila(at)huawei(dot)com> writes:
> We cannot directly compare expressions in target list as even if expressions
> are equal, below node (ex. APPEND) will
> not do projection, and hence expr will not be evaluated.
I think this is nonsense. Whatever the tlist of the lower node is, that
is supposed to describe what it's going to return. That node might not
be able to do projection, but that doesn't mean that something
underneath it didn't. So I think this patch is missing a bet by not
accepting equal() expressions. Did you have a test case showing that
this wouldn't work?
regards, tom lane
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