BUG #6426: Complex query runs 10 times longer with "LIMIT 20"

The following bug has been logged on the website:

Bug reference: 6426
Logged by: Andrew Schetinin
Email address: aschetinin(at)gmail(dot)com
PostgreSQL version: 9.1.2
Operating system: Debian Linux
Description:

I have a complex query that, when unlimited, runs in 2.5 seconds and returns
400+ records.

The same query, with added "LIMIT 20 OFFSET 0" at the end, runs 25 seconds!
(and returns correct 20 records)

I think there is a bug in the query optimizer. Other variations of this
query (with slightly different constraints and joins) work fast with and
without LIMIT.

The same query works fast in PostgreSQL 8.4 (tested on another machine with
Ubuntu and not exactly the same but very-very similar database).

I can provide the query execution plans of the two cases (as long as they
are not published on the Web); but not the database because there is
customer's data in it.

Please let me know if anybody is interested in researching the problem.

Regards,

Andrew Schetinin