Re: [WIP Patch] Using 128-bit integers for sum, avg and statistics aggregates

On 10/28/2014 04:06 PM, Tom Lane wrote:
> Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> writes:
>> It wouldn't be too hard to just do:
>
>> struct {
>> int64 high_bits;
>> uint64 low_bits;
>> } pg_int128;
>
>> and some macros for the + - etc. operators. It might be less work than
>> trying to deal with the portability issues of a native C datatype for this.
>
> -1. That's not that easy, especially for division, or if you want to
> worry about overflow.

The patch doesn't do division with the 128-bit integers. It only does
addition and multiplication. Those are pretty straightforward to implement.

> The point of this patch IMO is to get some low
> hanging fruit; coding our own int128 arithmetic doesn't sound like
> "low hanging" to me.

I wasn't thinking of writing a full-fledged 128-bit type, just the the
few operations needed for this patch.

> Also, we've already got the configure infrastructure for detecting
> whether a platform has working int64. It really shouldn't be much
> work to transpose that to int128 (especially if we don't care about
> printf support, which I think we don't).

It would be nicer to be able to use the same code on all platforms. With
a configure test, we'd still need a fallback implementation for
platforms that don't have it.

- Heikki