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

[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.

[jkatz]

@XuanYang-cn Does this require additional changes? Thanks!

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[alwayslove2013]

@jkatz Nice! Thank you for bringing this to our attention. We may need to recheck whether this has the same impact on other clients as well.

significant performance impact when converting a vector from binary => text => binary representation.

[jkatz]

@alwayslove2013 Agreed! Thank you for merging.