Re: Inlining comparators as a performance optimisation

On 21.09.2011 18:46, Tom Lane wrote:
> Andrew Dunstan<andrew(at)dunslane(dot)net> writes:
>> On 09/21/2011 10:50 AM, Tom Lane wrote:
>>> The other question that I'm going to be asking is whether it's not
>>> possible to get most of the same improvement with a much smaller code
>>> footprint. I continue to suspect that getting rid of the SQL function
>>> impedance-match layer (myFunctionCall2Coll etc) would provide most of
>>> whatever gain is to be had here, without nearly as large a cost in code
>>> size and maintainability, and with the extra benefit that the speedup
>>> would also be available to non-core datatypes.
>
>> Can we get a patch so we can do benchmarks on this?
>
> Well, we'd have to negotiate what the API ought to be. What I'm
> envisioning is that datatypes could provide alternate comparison
> functions that are designed to be qsort-callable rather than
> SQL-callable. As such, they could not have entries in pg_proc, so
> it seems like there's no ready way to represent them in the catalogs.
>
> The idea that I was toying with was to allow the regular SQL-callable
> comparison function to somehow return a function pointer to the
> alternate comparison function, so that the first comparison in a given
> sort run would be done the traditional way but then we'd notice the
> provided function pointer and start using that. It would not be too
> hard to pass back the pointer using FunctionCallInfoData.context, say.
> The downside is adding cycles to unoptimized cases to uselessly check
> for a returned function pointer that's not there. Perhaps it could be
> hacked so that we only add cycles to the very first call, but I've not
> looked closely at the code to see what would be involved.

You could have a new function with a pg_proc entry, that just returns a
function pointer to the qsort-callback.

Or maybe the interface should be an even more radical replacement of
qsort, not just the comparison function. Instead of calling qsort,
tuplesort.c would call the new datatype-specific sort-function (which
would be in pg_proc). The implementation could use an inlined version of
qsort, like Peter is suggesting, or it could do something completely
different, like a radix sort or a GPU-assisted sort or whatever.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com