Re: KNN-GiST with recheck

On Tue, Dec 16, 2014 at 4:37 PM, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com
> wrote:

> Patch attached. It should be applied on top of my pairing heap patch at
> http://www.postgresql.org/message-id/548FFA2C.7060000@vmware.com. Some
> caveats:
>
> * The signature of the distance function is unchanged, it doesn't get a
> recheck argument. It is just assumed that if the consistent function sets
> the recheck flag, then the distance needs to be rechecked as well. We might
> want to add the recheck argument, like you Alexander did in your patch, but
> it's not important right now.
>

I didn't get how that expected to work if we have only order by qual
without filter qual. In this case consistent function just isn't called at
all.

* I used the "distance" term in the executor, although the ORDER BY expr
> machinery is more general than that. The value returned by the ORDER BY
> expression doesn't have to be a distance, although that's the only thing
> supported by GiST and the built-in opclasses.
>
> * I short-circuited the planner to assume that the ORDER BY expression
> always returns a float. That's true today for knn-GiST, but is obviously a
> bogus assumption in general.
>
> This needs some work to get into a committable state, but from a
> modularity point of view, this is much better than having the indexam to
> peek into the heap.

Nice idea to put reordering into index scan node. Doesn't look like much of
overengineering. I'm going to bring it to more commitable state.

------
With best regards,
Alexander Korotkov.