Re: Inlining comparators as a performance optimisation

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