On Thu, Apr 20, 2017 at 3:17 PM, jonathan vanasco <postgres@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.
