Trying out read streams in pgvector (an extension)

Hi,

I was looking around for an exotic index type to try the experience of
streamifying an extension, ie out-of-core code.  I am totally new to
pgvector, but since everyone keeps talking about it, I could not avoid
picking up some basic facts in the pgconf.dev hallway track, and
understood that its scans have some degree of known-order access
predictability, and then also some degree of fuzzy-predictable
order-not-yet-determined access too.  It's also quite random in the
I/O sense.

Here's a toy to streamify the known-order part.  I think for the fuzzy
part that links those parts together, maybe there is some way to guess
when it's a reasonable time to speculatively prefetch the lowest order
stuff in the pairing heap, and then deal with it if you're wrong, but
I didn't try that...

Someone involved in that project mentioned that it's probably not a
great topic to research in practice, because real world users of HNSW
use fully cached ie prewarmed indexes, because the performance is so
bad otherwise.  (Though maybe that argument is a little circular...).
So although this patch clearly speeds up cold HSNW searches to a
degree controlled by effective_io_concurrency, I'll probably look for
something else.  Suggestions for interesting index types to look at
streamifying are very welcome!

Hmm.  If that's really true about HNSW though, then there may still be
an opportunity to do automatic memory prefetching[1].  But then in the
case of index building, "stream" is NULL in this patch anyway.  It
surely must also be possible to find some good places to put
profitable explicit pg_mem_prefetch() calls given the predictability
and the need to get only ~60ns ahead for that usage.  I didn't look
into that because I was trying to prove things about read_stream.c,
not get involved in another project :-D

Here ends my science experiment report, which I'm dropping here just
in case others see useful ideas here.  The main thing I learned about
the read stream API is that it'd be nice to be able to reset the
stream but preserve the distance (something that came up on the
streaming sequential scan thread for a different reason), to deal with
cases where look-ahead opportunities come in bursts but you want a
longer lived stream than I used here.  That is the reason the patch
creates and destroys temporary streams in a loop; doh.  It also
provides an interesting case study for what speculative random
look-ahead support might need to look like.

[1] https://www.postgresql.org/message-id/flat/CA%2BhUKGKNUMnqubrrz8pRBdEM8vHeSCZcNq7iqERmkt6zPtpA3g%40mail.gmail.com

=== setup ====

create extension vector;

create or replace function random_vector(dimensions int)
returns vector language sql
begin atomic;
  select array_agg(random())::vector
    from generate_series(1, dimensions);
end;

create table t (id serial, embedding vector(6));

insert into t (embedding)
select random_vector(6)
  from generate_series(1, 1000000);

set maintenance_work_mem = '2GB';

create index on t using hnsw(embedding vector_l2_ops);

=== test of a hot search, assuming repeated ===

select embedding <-> '[0.5,0.5,0.5,0.5,0.5,0.5]'::vector
  from t
 where embedding <-> '[0.5,0.5,0.5,0.5,0.5,0.5]'::vector < 0.2
 order by 1 limit 20;

=== test of a cold search, assuming empty caches ===

create or replace function test()
returns void
language plpgsql as
$$
declare
  my_vec vector(6) := random_vector(6);
begin
  perform embedding <-> my_vec
     from t
    where embedding <-> my_vec < 0.2
    order by 1 limit 20;
end;
$$;

select test();

(Make sure you remember to set effective_io_concurrency to an
interesting number if you want to generate a lot of overlapping
fadvise calls.)