BUG #5230: Limit operator slows down

The following bug has been logged online:

Bug reference: 5230
Logged by: aftab
Email address: akhangd(at)hotmail(dot)co(dot)uk
PostgreSQL version: 8.3.8
Operating system: Centos 5
Description: Limit operator slows down
Details:

S1="SELECT *
FROM position WHERE
position.POSITION_STATE_ID=2 AND
position.TARGET_ID=18
ORDER BY position.ID DESC
";

S2="SELECT *
FROM position WHERE
position.POSITION_STATE_ID=2 AND
position.TARGET_ID=18
ORDER BY position.ID DESC
LIMIT 1
";

S1 takes 0.16ms compared to S2 which takes 5 secs. Both S1 and S2 are same
except "LIMIT 1 " is added to S2.

Query Plan for S1:

QUERY PLAN
----------------------------------------------------------------------------
-------------------------------------------------------------
Sort (cost=209341.40..209767.55 rows=170457 width=297) (actual
time=0.200..0.200 rows=1 loops=1)
Sort Key: id
Sort Method: quicksort Memory: 25kB
-> Bitmap Heap Scan on "position" (cost=5050.22..146754.50 rows=170457
width=297) (actual time=0.188..0.189 rows=1 loops=1)
Recheck Cond: (target_id = 18)
Filter: (position_state_id = 2)
-> Bitmap Index Scan on position_target_fk (cost=0.00..5007.61
rows=170912 width=0) (actual time=0.134..0.134 rows=2 loops=1)
Index Cond: (target_id = 18)
Total runtime: 0.275 ms
(9 rows)

Query Plan for S2:

QUERY PLAN
----------------------------------------------------------------------------
----------------------------------------------------------------------------
---
Limit (cost=0.00..2.43 rows=1 width=297) (actual time=3053.220..3053.221
rows=1 loops=1)
-> Index Scan Backward using position_pkey on "position"
(cost=0.00..413713.28 rows=170459 width=297) (actual time=3053.216..3053.216
rows=1 loops=1)
Filter: ((position_state_id = 2) AND (target_id = 18))
Total runtime: 3053.297 ms
(4 rows)