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Re: Inlining comparators as a performance optimisation

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Inlining comparators as a performance optimisation
Date: 2011-12-06 21:23:17
Message-ID: 15713.1323206597@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Dec 6, 2011 at 1:07 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I'll take another crack at it.  I'm not entirely sold yet on merging
>> the two structs; I think first we'd better look and see what the needs
>> are in the other potential callers I mentioned.  If we'd end up
>> cluttering the struct with half a dozen weird fields, it'd be better to
>> stick to a minimal interface struct with various wrapper structs, IMO.

> OK.  I'll defer to whatever you come up with after looking at it.

OK, it looks like nodeMergeAppend.c could use something exactly like the
draft SortKey struct, while nodeMergejoin.c could embed such a struct in
MergeJoinClauseData.  The btree stuff needs something more nearly
equivalent to a ScanKey, including a datum-to-compare-to and a flags
field.  I'm inclined to think the latter would be too specialized to put
in the generic struct.  On the other hand, including the reverse and
nulls_first flags in the generic struct is clearly a win since it allows
ApplyComparator() to be defined as a generic function.  So the only
thing that's really debatable is the attno field, and I'm not anal
enough to insist on a separate level of struct just for that.

I am however inclined to stick with the shortened struct name SortSupport
rather than using SortKey.  The presence of the function pointer fields
(especially the inlined-qsort pointers, assuming we adopt some form of
Peter's patch) changes the struct's nature in my view; it's not really
describing just a sort key (ie an ORDER BY column specification).

			regards, tom lane

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