On Thu, Apr 20, 2017 at 3:17 PM, jonathan vanasco <postg...@2xlp.com> wrote:
> > 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. > > > There is no requirement in this query that bar_id be in the example_a__rollup table and since it is only in one table it is unambiguous so the server doesn't complain. It may be explanatory to add a couple records to your example_a_rollup table: insert into example_a__data values (3,4),(5,6); Then run a simple select showing what the where clause would see: SELECT, foo_id, (SELECT bar_id FROM example_a__rollup) FROM example_a__data ; foo_id | bar_id --------+-------- 3 | 5 | bar_id is null because there are no rows in example_a_rollup. Now add a single record to example_a_rollup: insert into example_a__rollup (id) values (10); Rerun the query and you will get: foo_id | bar_id --------+-------- 3 | 4 5 | 6 If you add another record to example_a__rollup and run it and you will get: ERROR: more than one row returned by a subquery used as an expression Although the subquery won't work as an expression it would still work in a the where clause but I doubt it will return what you desire. Unfortunately there are lots of ways to write syntactically correct but logically flawed statements. Cheers, Steve