why isn't this subquery wrong?

[jonathan vanasco <postgres@xxxxxxxx>]

I ran into an issue while changing a database schema around.  Some queries still worked, even though I didn't expect them to.

Can anyone explain to me why the following is valid (running 9.6) ?

schema

CREATE TEMPORARY TABLE example_a__data (

foo_id INT,

bar_id INT

);

CREATE TEMPORARY TABLE example_a__rollup_source (

id int primary key,

name varchar(64),

foo_id INT,

check_bool BOOLEAN

);

CREATE TEMPORARY TABLE example_a__rollup AS

SELECT id, name, foo_id

FROM example_a__rollup_source

WHERE check_bool IS TRUE

;

query:

SELECT foo_id

FROM example_a__data

WHERE foo_id IN (SELECT bar_id FROM example_a__rollup)

;

a raw select of `SELECT bar_id FROM example_a__rollup;` will cause an error because bar_id doesn't exist

postgres doesn't raise an error because example_a__data does have a bar_id -- but example_a__rollup doesn't and there's no explicit correlation in the query.

can someone explain why this happens?  i'm guessing there is a good reason -- but I'm unfamiliar with the type of implicit join/queries this behavior is enabling.