Re: Aggregate versions of hashing functions (md5, sha1, etc...)

On Fri, Jul 11, 2025 at 5:01 AM Dominique Devienne <ddevienne(at)gmail(dot)com>
wrote:

> On Thu, Jul 10, 2025 at 7:11 PM Ron Johnson <ronljohnsonjr(at)gmail(dot)com>
> wrote:
> > On Thu, Jul 10, 2025 at 12:26 PM Adrian Klaver <
> adrian(dot)klaver(at)aklaver(dot)com> wrote:
> >> On 7/10/25 04:48, Dominique Devienne wrote:
> >> > Seems so logical to me, that these hashing functions were available
> >> > are aggregates, I can't be the first one to think of that, can it?
> >>
> >> I've been on this list since late 2002 and I don't recall this ever
> >> being brought up. Now it is entirely possible that age has dimmed my
> >> recall abilities:) Though a quick search seems to confirm my memory.
> >
> > What even is an aggregate hash function? (I can imagine a few
> possibilities, but don't want to assume.)
>
> Well, it's so obvious to me, I wonder if you're baiting me :)
>
> Any hasher/digest inits some internal state, processes bytes,
> typically in "streaming-fashion" via successive byte spans (equivalent
> to PostgreSQL's bytea), and yields a digest of various length at the
> end. The current md5() and pgcrypto.digest() functions roll the x1
> init, xN process, and x1 finish into a single call, processing a
> single bytea (or perhaps more intelligently for TOAST'ed values, the
> 2K "rows" of those in streaming-fashion, hopefully. Can a dev
> confirm?). As an aggregate, the processing is extended to all values
> aggregated.

So it "appends" all the fields into one (virtual) mega-structure and takes
the hash on that.

It's what I expected but wanted to verify.

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