Casting a Vector to HalfVec with the pgvector extension

[A2va]

Backend

PostgreSQL

Diesel version

2.3.2

Diesel Features

postgres

Operating System Version

Fedora Linux 42 (COSMIC) x86_64

Third party libraries

No response

What do you want to do?

I'm currently implementing the conversion of a vector to a half-vector using the pgvector extension.
Taken this sql query SELECT * FROM items ORDER BY embedding::halfvec(3072) <=> '[1,2,3]' LIMIT 5;

I managed to output this SELECT "items"."id", "items"."embedding" FROM "items" ORDER BY CAST("items"."embedding" AS halfvec) <-> $1 LIMIT $2 with diesel. The issue is that the cast need the dimension of the vector (confirmed by the maintainer) as shown in the first query.

It is possible to achieve with diesel ?

Compile time error

No response

What code do you already have?

impl KnownCastSqlTypeName<Pg> for HalfVectorType {
    const SQL_TYPE_NAME: &'static str = "halfvec";
}

impl FallibleCastsTo<HalfVectorType> for Vector {}
impl CastsTo<HalfVectorType> for Vector {}

The work in question is here:
pgvector/pgvector-rust#22

Additional details

No response

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

[weiznich]

Yes that's not supported by the built-in cast operator. You should likely define your own query DSL extension for that. The code generated by diesel::infix_operator! might be a good starting point for that, although that macro also cannot completely handle this case out of the box.