No Index-Only Scan on Partial Index

Hackers,

I was trying to figure out why a query was not doing an index-only scan on a partial index, when Josh Berkus pointed to
thisissue, reported by Merlin Moncure: 
 http://www.postgresql.org/message-id/CAHyXU0x1OGao48WajAfUsbXqkUDLf=_6ho6hLMB8DSfkWdaVVw@mail.gmail.com

In short, the planner needs the column from the where clause included in the index to decide it can do an index-only
scan.This test case demonstrates the truth of this finding: 
   CREATE TABLE try (       id     INT NOT NULL,       label  TEXT NOT NULL,       active BOOLEAN DEFAULT TRUE   );
   INSERT INTO try   SELECT i        ,  (ARRAY['foo','bar','baz','wig'])[floor((random()*4))::int + 1]        , (i %
100)= 0     FROM generate_series(1, 100000) i; 
   VACUUM FREEZE TRY;
   CREATE INDEX idx_try_active ON try(id) WHERE active;
   -- Does a bitmap heap scan.   EXPLAIN (ANALYZE, FORMAT YAML)   SELECT id FROM try WHERE active;
   DROP INDEX idx_try_active;   CREATE INDEX idx_try_active ON try(label, id, active) WHERE active;
   -- Does an index-only scan.   EXPLAIN (ANALYZE, FORMAT YAML)   SELECT id FROM try WHERE active;
   DROP TABLE try;

The first query does a bitmap heap scan, but after the index that includes the active column is added, it does an
index-onlyscan. 

However, this does not quite match my case. In my case, I'm using an immutable function call in the index where clause:
   CREATE INDEX idx_try_active ON try(id, upper_inf(irange)) WHERE upper_inf(irange);

I am unable to get the planner do to an index-only scan with this index no matter what I do. Here’s the full test case:
   CREATE TABLE try (       id     INT       NOT NULL,       label  TEXT      NOT NULL,       irange INT4RANGE NOT NULL
 ); 
   INSERT INTO try   SELECT i        ,  (ARRAY['foo','bar','baz','wig'])[floor((random()*4))::int + 1]        ,
int4range(1,CASE WHEN random() < 0.01 THEN NULL ELSE 2 END)     FROM generate_series(1, 100000) i; 
   VACUUM FREEZE TRY;
   CREATE INDEX idx_try_active ON try(id) WHERE upper_inf(irange);
   -- Does a bitmap heap scan.   EXPLAIN (ANALYZE, FORMAT YAML)   SELECT id FROM try WHERE upper_inf(irange);
   DROP INDEX idx_try_active;   CREATE INDEX idx_try_active ON try(label, id, upper_inf(irange)) WHERE
upper_inf(irange);
   -- Also does a bitmap heap scan.   EXPLAIN (ANALYZE, FORMAT YAML)   SELECT id FROM try WHERE upper_inf(irange);
   DROP TABLE try;

So is there something about using a function in a conditional index that prevents index-only scans? Tested on 9.2 and
9.3,BTW. 

Thanks,

David