Re: KNN-GiST with recheck

I added the point to polygon distance operator patch to the open
CommitFest as ready for committer and added myself as reviewer to
both of the patches.

> I think that for most use cases just some operators require further sorting
> and some of them not. But it could appear one day that some index gives
> part of its knn answers exact and part of them inexact. Same happen to
> recheck of regular operators. Initially recheck flag was defined in
> opclass. But later recheck became runtime flag.

I cannot think of an use case, but it makes sense to add the flag to
the distance function just like the consistent function if we will go
with this implementation.

> Cost estimation of GiST is a big problem anyway. It doesn't care (and
> can't) about amount of recheck for regular operators. In this patch, same
> would be for knn recheck. The problem is that touching heap from access
> method breaks incapsulation. One idea about this is to do sorting in
> another nodes. However, I wonder if it would be an overengineering and
> overhead. In attached patch I propose a different approach: put code
> touching heap into separate index_get_heap_values function. Also new
> version of patch includes regression tests and some cleanup.

While looking it at I found a bug.  It returns the second column
in wrong order when both of the distance functions return recheck = true.
Test script attached to run on the regression database.  I tried to
fix but could not.  searchTreeItemDistanceRecheck function is not
very easy to follow.  I think it deserves more comments.