| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, David Fetter <david(at)fetter(dot)org>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: LATERAL |
| Date: | 2009-09-23 03:21:57 |
| Message-ID: | 12171.1253676117@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I've been turning this one over in my head. It seems to me that this
> is very similar to what we already do with inner index-scans, only
> generalized to joinrels.
Right.
> Currently, however, we only consider this possibility when the inner
> rel is NOT a joinrel. It seems like it might be possible to change
> this, but it doesn't look straightforward.
Well, it's straightforward enough in theory, it's just the exponential
growth in the number of join paths to consider that's a problem :-(.
I think what we'd need is a heuristic to limit the paths considered.
I think Andrew was suggesting that we not attempt to consider this
automatically, but only when the user writes the query in a way that
exposes it directly via LATERAL. While that goes against my normal
instincts for the planner, it isn't unreasonable as a first step.
regards, tom lane
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