Ah, right. For the kNN/GiST support in Postgres, distance has to be defined in such a way that it is computable given only two data values; furthermore, I'm p

Ah, right. For the kNN/GiST support in Postgres, distance has to be defined in such a way that it is computable given only two data values; furthermore, I'm pretty sure it has to be both symmetric and satisfy the triangle inequality in order for the index to produce correct results.