Grouping sets

[Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>]

Hello

I got a possibility to continue on GROUPING SETS see
http://wiki.postgresql.org/wiki/Grouping_Sets

My last patch was based on an sharing functionality with non recursive
CTE. I would to recapitulate all options here and I would to prepare
this patch for next commitfest.

a) using actualised patch - review:
http://archives.postgresql.org/message-id/162867790811240316y52227d88xe53527399b329e05(at)mail(dot)gmail(dot)com
  + it is simple (share lot of code)
  + it is near to natural implementation - result is in "good" order
  + allows full implementation of ANSI SQL syntax - GROUP BY
[>>ALL<<|DISTINCT] GROUPING SETS(..
  - is serial
  - I read some objections - "it is only syntactic sugar"

b) modification of a (directly using tuple store without CTE code)
  + add some new code
  - it isn't syntactic sugar

c) go back to my first patch and use "feeders"
  - add some complexity to GROUP BY planner ( :-( )
  +/- add new concept to PostgreSQL - semi parallel execution
  - when datatype isn't hashable, then have to use serial processing
  - have to solve some corner behave of our hashtable support
  - should be very fast

I still prefer option a (with some cleaning)

I invite any comments and helpers.

Regards
Pavel Stehule