Re: KNNGiST for knn-search (WIP)

>> Done, IndexScan and IndexPath have separate field to store order clauses.
> 
> Why?  Isn't that redundant with the pathkey structures?  We generate
> enough paths during a complex planning problem that I'm not in favor
> of adding unnecessary structures to them.
I found two ways to add support of knn-seaech to planner
- 0.4 version: add sorting clauses to restrictclauses in find_usable_indexes,
   and there is two issues:
   - find_usable_indexes could not be used to find indexes for index and bitmap
     scans at once, because sorting clauses will be used in bitmap scan. Full
     scan of index with knn-ordering on large index is much more expensive.
   - implied/refused predicate machinery is teached to ignore sorting clauses,
     but it's not so obvious: it should ignore operation returning non-boolean
     values
- 0.4.1 version: pull sort clauses separately and merge them with regular
   clauses at create_indexscan_plan function. That's solves problems above

Do you suggest to construct that clauses from pathkey representation? And append 
  constructed clauses to indexquals in create_indexscan_plan?
-- 
Teodor Sigaev                                   E-mail: teodor@sigaev.ru
                                                    WWW: http://www.sigaev.ru/