PostgreSQL subquery with syntax error gives a valid result

Question

What is happening here?

I got two tables, test1 and test2:

create table test1 (id1 int4 primary key);
create table test2 (id2 int4 primary key);


        

Answer 1

Columns from outer select are visible in sub-select.

Your query is equivalent to:

select * 
from test1 
where test1.id1 in (select test1.id1 from test2);