BUG #13880: Different procedures in same SQL

The following bug has been logged on the website:

Bug reference:      13880
Logged by:          Victor Neves
Email address:      victor.evangelista@caixa.gov.br
PostgreSQL version: 9.1.9
Operating system:   RedHat 4.4.7 x64
Description:

I found a different procedures in same SQL, in postgres 9.1.9.

I create a table, but i dont insert any data, after execute

insert into <table> (namecolum1, namecolum2, namecolum3, namecolum4)
select distinct 'value1', 'value2', 'value3', 'value4'
from <table> where not exists (select 1 from <table> sis where
sis.namecolum2='value2' )

Postgres says: "Query returned successfully: 0 rows affected, 1 ms execution
time."

But if I insert any data in the table and execute the SQL again, the insert
works fine!

If I truncate the table and execute de SQL, postgres says "0 rows affected"
again.