pgvector: ensure vector is sent in binary representation

[jkatz]

PostgreSQL supports two methods of passing data from client to server: text and binary. While for many data types the difference may not be noticeable, we can see significant performance impact when converting a vector from binary => text => binary representation. See previous explanation here[1].

While the pgvector loading code accounts for this, the query code did not. This is due to the use of a list[float] type, which the pgvector-python adapter currently doesn't support. However, this adapter does support direct binary transfer if the data is represent as a Numpy array[2]. Testing shows that moving to a direct binary representation does have a significant impact on query results - my tests are showing a 3x impact -- and provides a more accurate representation for how this workload would execute.

[1] erikbern/ann-benchmarks#488
[2] https://github.com/pgvector/pgvector-python?tab=readme-ov-file#psycopg-3

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[jkatz]

/assign @XuanYang-cn

[jkatz]

Updating with some additional testing: the speedup tends to impact queries that have higher throughput, e.g. for HNSW, lower values of hnsw.ef_search. This is due to the high throughput rate and reducing the overhead of converting from binary/text/binary. At higher values of hnsw.ef_search, the impact is less noticeable as more time is spent on the index lookups.