From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Tom Raney <raneyt(at)cecs(dot)pdx(dot)edu> |
Cc: | Postgres Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Planner question |
Date: | 2008-09-10 18:41:16 |
Message-ID: | 4159.1221072076@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Tom Raney <raneyt(at)cecs(dot)pdx(dot)edu> writes:
> Why does the index scan for tenk1 include a path key from
> onek.unique2? Is it implying an equivalence there?
> bench=# explain select * from tenk1 JOIN onek ON
> tenk1.unique2=onek.unique2;
Yes, for an example like that the planner knows that tenk1.unique2 and
onek.unique2 will have equal values in any valid join row, so it's okay
to suppose that a sort by one is the same as a sort by the other. So
the pathkey items actually reference sets of variables
{tenk1.unique2, onek.unique2}
not just individual variables.
> RELOPTINFO (tenk1): rows=10000 width=244
> path list:
> SeqScan(tenk1) rows=10000 cost=0.00..434.00
> IdxScan(tenk1) rows=10000 cost=0.00..583.25
> pathkeys: ((tenk1.unique2, onek.unique2)) <---
> cheapest startup path:
> SeqScan(tenk1) rows=10000 cost=0.00..434.00
> cheapest total path:
> SeqScan(tenk1) rows=10000 cost=0.00..434.00
Hm, I don't recognize this output format, is it coming from some custom
code?
regards, tom lane
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