BUG #5226: Limit operator slows down

The following bug has been logged online:

Bug reference:      5226
Logged by:          aftab
Email address:      akhangd@hotmail.co.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.