On Tue, Jan 28, 2014 at 9:32 PM, Heikki Linnakangas <hlinnakan...@vmware.com > wrote:
> On 01/28/2014 04:12 PM, Alexander Korotkov wrote: > >> On Tue, Jan 28, 2014 at 5:54 PM, Heikki Linnakangas < >> hlinnakan...@vmware.com >> >>> wrote: >>> >> >> 4. (as you mentioned in the other thread: ) It's a modularity violation >>> that you peek into the heap tuple from gist. I think the proper way to do >>> this would be to extend the IndexScan executor node to perform the >>> re-shuffling of tuples that come from the index in wrong order, or >>> perhaps >>> add a new node type for it. >>> >>> Of course that's exactly what your partial sort patch does :-). I haven't >>> looked at that in detail, but I don't think the approach the partial sort >>> patch takes will work here as is. In the KNN-GiST case, the index is >>> returning tuples roughly in the right order, but a tuple that it returns >>> might in reality belong somewhere later in the ordering. In the partial >>> sort patch, the "input stream" of tuples is divided into non-overlapping >>> groups, so that the tuples within the group are not sorted, but the >>> groups >>> are. I think the partial sort case is a special case of the KNN-GiST >>> case, >>> if you consider the lower bound of each tuple to be the leading keys that >>> you don't need to sort. >>> >> >> Yes. But, for instance btree accesses heap for unique checking. Is really >> it so crimilal? :-) >> > > Well, it is generally considered an ugly hack in b-tree too. I'm not 100% > opposed to doing such a hack in GiST, but would very much prefer not to. > > > This is not only question of a new node or extending existing node. We >> need >> to teach planner/executor access method can return value of some >> expression >> which is lower bound of another expression. AFICS now access method can >> return only original indexed datums and TIDs. So, I afraid that enormous >> infrastructure changes are required. And I can hardly imagine what they >> should look like. >> > > Yeah, I'm not sure either. Maybe a new field in IndexScanDesc, along with > xs_itup. Or as an attribute of xs_itup itself. This shouldn't look like a hack too. Otherwise I see no point of it: it's better to have some isolated hack in access method than hack in planner/executor. So I see following changes to be needed to implement this right way: 1) Implement new relation between operators: operator1 is lower bound of operator2. 2) Extend am interface to let it return values of operators. 3) Implement new node for knn-sorting. However, it requires a lot of changes in PostgreSQL infrastructure and can appear to be not enough general too (we don't know until we have another application). ------ With best regards, Alexander Korotkov.
