Re: [PATCH] Negative Transition Aggregate Functions (WIP)

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On 2014-01-16 14:23:47 -0500, Tom Lane wrote:
>> Dunno, I think that a transition state containing both an int64 and
>> a (presumably separately palloc'd) numeric will be a real PITA.

> Yea, not sure myself. I just dislike the idea of having a good part of a
> 128bit math implementation for a single transition function.

Not sure how you figure that we need very much new code beyond the
overflow test.

> Well, you don't need to check the second variable for lots of
> operations. Say, the current sum is 0 and you add a -1. With the two
> variables scheme that requires checking the second variable,
> manipulating it etc.

I'm envisioning just

state->lowhalf += input;
if (overflowed_up)
state->highhalf++;
else if (overflowed_down)
state->highhalf--;

The only thing that might take a moment's thought, or extra cycles in the
normal case, is extending the overflow test so that it can tell whether
we need to increment or decrement the upper half.

[ thinks a bit... ] Note that I'm supposing that the state is defined
as (highhalf * 2^64) + lowhalf, not that we need the two variables to
be exactly a 128-bit twos-complement value, which is what I think
you're talking about.

regards, tom lane