Strange browser behavior when querying PostgreSql database

Hi,

I have a problem in a php script :

when I execute it via Internet Explorer , sometimes the source is
complete (no problem), sometimes the source is incomplete (the html
page is cutted), and sometimes the browser doesn't display anything
and wait and wait and wait ....

The problem comes from a request I peform in this PHP script :

when I do the following request I have the problems quoted above :

SELECT a.* FROM a WHERE a.idf = b.id

I solved the problem by modifying the request :

SELECT a.* FROM a, b WHERE a.idf = b.id

Now the script works.

But I don't understand why the first request causes those problems ?

Is the first request has a correct syntax ? ( when submitted to
Postgres, there is no error message , and the results are displayed)
Is it due to PHP ? or PostgreSQL?

Thanks in advance,

Christophe Gras