SELECT query not using pgvector (HNSW) index

Question

Given a table of about 1 mil. rows with columns id of type integer and embedding of type vector(2000), I ran the following query in pgAdmin query tool:

CREATE INDEX ON table USING hnsw (embedding vector_cosine_ops);

Whatever the SELECT query I ran after, I'm not seeing the index being used when prefixing the query with EXPLAIN ANALYZE.

After seeing @ankane's comments "ordering by an expression [...] won't use the index" and "Postgres only supports ASC order index scans on operators", I tried a simple SELECT id, embedding <=> $1 FROM table but it's still not using the index.

Many blog articles such as this GCP one and this other one recommend to use EXPLAIN to check if the index is used.

Thus my following questions:

• How can I check that the index is actually built? With EXPLAIN I can see that even with the simplest query does not use the index, thus I'm even wondering if the index has actually been created (despite the CREATE INDEX having "returned successfully in 1 hr 6 min.")
• How to use the HNSW index with a SELECT statement?
• Assuming an index has been created, what happens if I rerun a CREATE INDEX statement? If I create an HNSW index, I assume it overwrites the existing index (is it?), does it overwrite if I create an IVFFLAT index instead?

Answer 1

I believe your issue could be because you are not using ORDER BY with LIMIT. Under the querying section there is this note...

Note: Combine with ORDER BY and LIMIT to use an index

pgvector Querying Readme