BUG #2208: Low performance on select

The following bug has been logged online:

Bug reference:      2208
Logged by:          Fahri CAKIROGLU
Email address:      fahri@gbks.com.tr
PostgreSQL version: 8.1
Operating system:   windows nt 2000
Description:        Low performance on select
Details:

Consecutive selects from three different tables
causes low performance(about 1500 ms).
Same select count from two tables gives good performance(about 30 ms).
All searched keys are indexed and individual execution
of each query is very fast.

Slow query :
select * from orders where ordernum='1' order by ordernum limit 1;
select * from customers where custnum='1' order by custnum limit 1;
select * from invoice where invnum='1' order by inv limit 1;

Query with good performance :
select * from orders where ordernum='1' order by ordernum limit 1;
select * from customers where custnum='1' order by custnum limit 1;
select * from orders where ordernum='2' order by ordernum limit 1;
select * from customers where custnum='2' order by custnum limit 1;