Why prefilter_source does not limit the scanned fragments to be those covered by a vector index?

[niebayes]

I've noticed that the Scanner::prefilter_source method always scans all fragments in the dataset, either through scalar indexed scan or a full scan. I wonder if it's reasonable to limit the scanned fragments to be those covered by a vector index.

[westonpace]

Agree that it should do that. I have that change here which is part of a larger overhaul of how scans as source work but I'm not rushing to get it merged in just yet as I want to make sure I'm not breaking any existing read performance first by changing things.

[niebayes]

Besides the performance, I'm worrying about the correctness. The topk from all fragments may not be the topk from the fragments covered by the vector index.