Function private_left_join_multiple_keys 

pub fn private_left_join_multiple_keys(
    a: &[Column],
    b: &[Column],
    nulls_equal: bool,
) -> Result<(Either<Vec<u32>, Vec<ChunkId>>, Either<Vec<NullableIdxSize>, Vec<ChunkId>>), PolarsError>

Available on crate feature polars-ops only.