[HACKERS] [PATCH] Incremental sort (was: PoC: Partial sort)

Hi all!

I decided to start new thread for this patch for following two reasons.

 * It's renamed from "Partial sort" to "Incremental sort" per suggestion by Robert Haas [1].  New name much better characterizes the essence of algorithm.

 * I think it's not PoC anymore.  Patch received several rounds of review and now it's in the pretty good shape.

Attached revision of patch has following changes.

 * According to review [1], two new path and plan nodes are responsible for incremental sort: IncSortPath and IncSort which are inherited from SortPath and Sort correspondingly.  That allowed to get rid of set of hacks with minimal code changes.

 * According to review [1] and comment [2], previous tuple is stored in standalone tuple slot of SortState rather than just HeapTuple.

 * New GUC parameter enable_incsort is introduced to control planner ability to choose incremental sort.

 * Test of postgres_fdw with not pushed down cross join is corrected.  It appeared that with incremental sort such query is profitable to push down.  I changed ORDER BY columns so that index couldn't be used.  I think this solution is more elegant than setting enable_incsort = off.

Also patch has set of assorted code and comments improvements.

Links

1. https://www.postgresql.org/message-id/CA+TgmoZapyHRm7NVyuyZ+yAV=U1a070BOgRe7PkgyrAegR4JDA@mail.gmail.com

2. https://www.postgresql.org/message-id/CAM3SWZQL4yD2SnDheMCGL0Q2b2oTdKUvv_L6Zg_FcGoLuwMffg@mail.gmail.com