Index Scan Backward on wrong index in partitioned table.

Hi ,

We have a set of partitioned tables and we run the query on main table
the query is

select uniq_id ,profile_id  from general.profile_log where
profile_id=3528336 order by uniq_id desc limit 5;

there is a index on profile_id on the child tables of profile_log. The
query on profile_id does not chooses that
index instead it uses an index on uniq_id ( the sorting column).

Since the number of child table is huge i am only posting an excerpt
of the plan .


                ----------------------------------------------------------------
                     Filter: (profile_id = 3528336)
               ->  Index Scan Backward using profile_log_2011_08_pkey
on profile_log_2011_08 profile_log  (cost=0.00..15815.11 rows=3
width=8)
                     Filter: (profile_id = 3528336)
               ->  Index Scan Backward using profile_log_2011_09_pkey
on profile_log_2011_09 profile_log  (cost=0.00..17851.91 rows=76
width=8)

---------------------------------------------------------------------
the index profile_log_2011_09_pkey  is the index on the column uniq_id

the query is *very slow* , 39 seconds


how ever if we just change  limit 5 to limit 15 then the plan changes

   Index Cond: (profile_id = 3528336)
                     ->  Bitmap Heap Scan on profile_log_2011_08
profile_log  (cost=4.31..16.13 rows=3 width=8)
                           Recheck Cond: (profile_id = 3528336)
                           ->  Bitmap Index Scan on
profile_log_2011_08_profile_id  (cost=0.00..4.31 rows=3 width=0)
                                 Index Cond: (profile_id = 3528336)
                     ->  Bitmap Heap Scan on profile_log_2011_09
profile_log  (cost=4.89..285.93 rows=76 width=8)
                           Recheck Cond: (profile_id = 3528336)
                           ->  Bitmap Index Scan on
profile_log_2011_09_profile_id  (cost=0.00..4.87 rows=76 width=0)
                                 Index Cond: (profile_id = 3528336)

 the index on profile_id is being used and the query is very fast ( 50 ms)


I am using the most recent released version of postgresql at this
moment which is 9.1.1


can any one please suggest , I think autovaccum is on.


regds
mallah