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From: Ron Johnson <[email protected]>
To: pgsql-general <[email protected]>
Subject: Re: Aggregate versions of hashing functions (md5, sha1, etc...)
Date: Fri, 11 Jul 2025 07:46:45 -0400
Message-ID: <CANzqJaD9kq9E37oqv+NZ3JcGweOg7Eij2OCsoberEoNSdyAoxQ@mail.gmail.com> (raw)
In-Reply-To: <CAFCRh-8chpiFs1oOGnOS=wTRd9y0t4cojv2iMwr0Zgo+j1RYTg@mail.gmail.com>
References: <CAFCRh-8FOZiycyfX4uPB8MTHQTxqNVuW0pdKBuFNQneEZy1PwQ@mail.gmail.com>
	<[email protected]>
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	<CAFCRh-8chpiFs1oOGnOS=wTRd9y0t4cojv2iMwr0Zgo+j1RYTg@mail.gmail.com>

On Fri, Jul 11, 2025 at 5:01 AM Dominique Devienne <[email protected]>
wrote:

> On Thu, Jul 10, 2025 at 7:11 PM Ron Johnson <[email protected]>
> wrote:
> > On Thu, Jul 10, 2025 at 12:26 PM Adrian Klaver <
> [email protected]> 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.

-- 
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!

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