Re: GIN improvements part 3: ordering in index

attached patch implementing ordering inside GIN index. This is third patch of GIN improvements, see previous two:

http://www.postgresql.org/message-id/CAPpHfduxv-iL7aedwPW0W5fXrWGAKfxijWM63_hZujaCRxnmFQ@mail.gmail.com
http://www.postgresql.org/message-id/CAPpHfdvftaJq7www381naLw1=4u0h+qpXgWvNhcEB9HMVywbGg@mail.gmail.com

This patch introduces new interface method of GIN which takes same arguments as consistent but returns float8. 
float8 gin_ordering(bool check[], StrategyNumber n, Datum query, int32 nkeys, Pointer extra_data[], bool *recheck, Datum queryKeys[], bool nullFlags[], Datum addInfo[], bool addInfoIsNull[])

This patch implements gingettuple method which can return ordering data using KNN infrastructure. Also it introduces >< operator for fts which support ordering in GIN index. Some example:

postgres=# explain analyze select * from dblp_titles2 where tsvector @@ to_tsquery('english', 'statistics') order by tsvector >< to_tsquery('english', 'statistics') limit 10;

                                                                   QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------------------

 Limit  (cost=12.00..48.22 rows=10 width=136) (actual time=6.999..7.120 rows=10 loops=1)

   ->  Index Scan using dblp_titles2_idx on dblp_titles2  (cost=12.00..43003.03 rows=11868 width=136) (actual time=6.996..7.115 rows=10 loops=1)

         Index Cond: (tsvector @@ '''statist'''::tsquery)

         Order By: (tsvector >< '''statist'''::tsquery)

 Total runtime: 7.556 ms

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