Re: Aggregate versions of hashing functions (md5, sha1, etc...) - Mailing list pgsql-general

On Thu, Jul 10, 2025 at 7:11 PM Ron Johnson <ronljohnsonjr@gmail.com> wrote:
> On Thu, Jul 10, 2025 at 12:26 PM Adrian Klaver <adrian.klaver@aklaver.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.