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Re: best way to fetch next/prev record based on index

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)com>,Merlin Moncure <merlin(dot)moncure(at)rcsonline(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: best way to fetch next/prev record based on index
Date: 2004-07-28 16:56:21
Message-ID: 23584.1091033781@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Greg Stark <gsstark(at)mit(dot)edu> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> The only reason the code in parse_expr.c appears new is that the
>> functionality used to be in gram.y.

> Ah, that was what I was missing. Though it's odd since it seems there was code
> in parse_expr.c to handle the "=" case specially.

IIRC, the case involving a subselect, eg
	... WHERE (1,2) = ANY (SELECT a, b FROM foo) ...
has always been handled in parse_expr.c, but cases involving simple
rows were previously expanded in gram.y.  One of the reasons I moved
the logic over to parse_expr.c was the thought that it would be easier
to do it right in parse_expr.c --- gram.y would not be allowed to look
up related operators, which seems necessary to handle the construct
per spec.

> I tried my hand at this last night and think I did an ok first pass.

The main issue in my mind is whether to invent a separate node type for
row comparisons.  This is probably a good idea for a number of reasons,
the most obvious being that there's no way to avoid multiple evaluations
of the subexpressions if you try to expand it into simple comparisons.
Also it seems likely that the planner would find it easier to recognize
the relationship to a multicolumn index than if the thing is expanded.
(But teaching the planner and the index mechanisms themselves about this
is going to be a major project in any case.)

One thing I did not like about your first pass is that it makes
unsupportable assumptions about there being a semantic relationship
between operators named, say, '<' and '<='.  Postgres used to have such
bogosity in a number of places but we've managed to get rid of most of
it.  (Offhand I think the only remaining hard-wired assumption about
operators of particular names having particular semantics is that the
foreign key mechanisms assume '=' must be the right operator to compare
keys with.  Eventually we need to get rid of that too.)

IMHO the right way to do this is to look up a suitable btree operator
class and use the appropriate member operators of that class.  (In a
separate-node-type implementation, we'd probably ignore the operators
as such altogether, and just call the btree comparison function of the
opclass.)  It's not entirely clear to me how to select the opclass when
the initially given inputs are of different types, though.  In the
present code we leave it to oper() to do the right thing, including
possibly coercing the inputs to matching types.  Possibly we should
still apply oper(), but then insist that the selected operator appear
as a btree opclass member, comparable to the way we handle sort
operators now.

			regards, tom lane

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