On Fri, Jul 11, 2025 at 5:01 AM Dominique Devienne <ddevienne@gmail.com> wrote:
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.
So it "appends" all the fields into one (virtual) mega-structure and takes the hash on that.