limit + order by is slow if no rows in result set - Mailing list pgsql-performance

There are 1.9M rows in ts_defects and indexes on b.ts_id (primary key)
d.ts_biz_event_id and d.ts_occur_date.  Both queries below return 0
rows.  The 1st runs fast and the 2nd > 400x slower.  The 2nd query
differs from the 1st only by the addition of "limit 1".

Why the big difference in performance?

Thanks,
Brian

[bcox@athena jsp]$ time PGPASSWORD=**** psql -U admin -d cemdb -h
192.168.1.30 -c 'select * from ts_defects d join ts_biz_events b on
b.ts_id = d.ts_biz_event_id where b.ts_status=3 order by d.ts_occur_date
desc;'
(column list deleted)

-------+--------------+--------------+---------------+---------------------------+----------------+----------------+------------+------------------------+------------------+-----------------+---------------+------------------+---------------+-----------------+-------------------+---------------------+---------------------+---------------------+--------------------+----------------+--------------------+----------------------+--------------------+----------------------+-----------------------+----------------+----------------------+---------------+-----------------+------------------+--------------+----------------+-------+--------------+---------------+---------------------------+------------------+---------+--------------------+---------------+-----------------+------------------+---------------+----------------------+---------------------+--------------------+-----------+---------------------+----------+---------------+--------------+------------------+-------------+--------
-----+--------------+--------------+----------------
(0 rows)


real    0m0.022s
user    0m0.003s
sys     0m0.003s


[bcox@athena jsp]$ time PGPASSWORD=**** psql -U admin -d cemdb -h
192.168.1.30 -c 'select * from ts_defects d join ts_biz_events b on
b.ts_id = d.ts_biz_event_id where b.ts_status=3 order by d.ts_occur_date
desc limit 1;'
(column list deleted)

-------+--------------+--------------+---------------+---------------------------+----------------+----------------+------------+------------------------+------------------+-----------------+---------------+------------------+---------------+-----------------+-------------------+---------------------+---------------------+---------------------+--------------------+----------------+--------------------+----------------------+--------------------+----------------------+-----------------------+----------------+----------------------+---------------+-----------------+------------------+--------------+----------------+-------+--------------+---------------+---------------------------+------------------+---------+--------------------+---------------+-----------------+------------------+---------------+----------------------+---------------------+--------------------+-----------+---------------------+----------+---------------+--------------+------------------+-------------+--------
-----+--------------+--------------+----------------
(0 rows)


real    0m9.410s
user    0m0.005s
sys     0m0.002s