Re: On disable_cost

On 8/21/24 10:29 AM, Robert Haas wrote:

> I went ahead and committed these patches. I know there's some debate
> over whether we want to show the # of disabled nodes and if so whether
> it should be controlled by COSTS, and I suspect I haven't completely
> allayed David's concerns about the initial_cost_XXX functions although
> I think that I did the right thing. But, I don't have the impression
> that anyone is desperately opposed to the basic concept, so I think it
> makes sense to put these into the tree and see what happens. We have
> quite a bit of time left in this release cycle to uncover bugs, hear
> from users or other developers, etc. about what problems there may be
> with this. If we end up deciding to reverse course or need to fix a
> bunch of stuff, so be it, but let's see what the feedback is.

We hit an issue with pgvector[0] where a regular `SELECT count(*) FROM 
table`[1] is attempting to scan the index on the vector column when 
`enable_seqscan` is disabled. Credit to Andrew Kane (CC'd) for flagging it.

I was able to trace this back to e2225346. Here is a reproducer:

Setup
=====

CREATE EXTENSION vector;

CREATE OR REPLACE FUNCTION public.generate_random_normalized_vector(dim 
integer)
RETURNS vector
LANGUAGE SQL
AS $$
     SELECT public.l2_normalize(array_agg(random()::real)::vector)
     FROM generate_series(1, $1);
$$;

CREATE TABLE test (id int, embedding vector(128));
INSERT INTO test
   SELECT n, public.generate_random_normalized_vector(128)
   FROM generate_series(1,5) n;

CREATE INDEX ON test USING hnsw (embedding vector_cosine_ops);

Test
====

SET enable_seqscan TO off;
EXPLAIN ANALYZE
SELECT count(*) FROM test;

Before e2225346:
----------------

Aggregate  (cost=10000041965.00..10000041965.01 rows=1 width=8) (actual 
time=189.864..189.864 rows
=1 loops=1)
    ->  Seq Scan on test  (cost=10000000000.00..10000040715.00 rows=5 
width=0) (actual time=0.01
8..168.294 rows=5 loops=1)
(4 rows)

With e2225346:
-------------
ERROR:  cannot scan hnsw index without order

Some things to note with the ivfflat/hnsw index AMs[3] in pgvector are 
that they're used for "ORDER BY" scans exclusively. They currently don't 
support index only scans (noting as I tried reproducing the issue with 
GIST and couldn't do so because of that), but we wouldn't want to do a 
full table "count(*)" on a IVFFlat/HNSW index anyway as it'd be more 
expensive than just a full table scan.

Thanks,

Jonathan

[0] https://github.com/pgvector/pgvector
[1] https://github.com/pgvector/pgvector/actions/runs/10519052945
[2] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=e2225346
[3] https://github.com/pgvector/pgvector/blob/master/src/hnsw.c#L192