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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: Peter Geoghegan <peter(at)2ndquadrant(dot)com>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Inlining comparators as a performance optimisation
Date: 2011-12-02 05:16:08
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Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Dec 1, 2011 at 10:48 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I am thinking that the btree code, at least, would want to just
>> unconditionally do
>>        colsortinfo->comparator(datum1, datum2, colsortinfo)
>> so for an opclass that fails to supply the low-overhead comparator,
>> it would insert into the "comparator" pointer a shim function that
>> calls the opclass' old-style FCI-using comparator.  (Anybody who
>> complains about the added overhead would be told to get busy and
>> supply a low-overhead comparator for their datatype...)  But to do
>> that, we have to have enough infrastructure here to cover all cases,
>> so omitting collation or not having a place to stash an FmgrInfo
>> won't do.

> I'm slightly worried about whether that'll be adding too much overhead
> to the case where there is no non-FCI comparator.  But it may be no
> worse than what we're doing now.

It should be the same or better.  Right now, we are going through
FunctionCall2Coll to reach the FCI-style comparator.  The shim function
would be more or less equivalent to that, and since it's quite special
purpose I would hope we could shave a cycle or two.  For instance, we
could probably afford to set up a dedicated FunctionCallInfo struct
associated with the SortSupportInfo struct, and not have to reinitialize
one each time.

			regards, tom lane

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