| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Greg Stark <gsstark(at)mit(dot)edu> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, 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: | 2009-10-19 22:26:14 |
| Message-ID: | 12669.1255991174@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Greg Stark <gsstark(at)mit(dot)edu> writes:
> nested loop
> index scan expecting 1 record
> merge join
> index scan on <col1,col2> where col1 = outer.foo and col2
> between a and b
> some other scan
> Ie, where the nested loop is a degenerate nested loop which only
> expects a single value and provides a parameter which allows some
> partial index to work or allows for some other index scan by providing
> a higher order key element?
Right. I don't see any particular reason to assume the inner path
is iterated only once, either. If the key value coming from the outer
path is sufficiently useful, this could be a win even with multiple
iterations, as compared to having to scan the whole of some large
relation or other ...
regards, tom lane
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