Re: BUG #16834: where-in: if the nested query fails, the parent query returns all the records

On Fri, Jan 22, 2021 at 2:38 PM Pantelis Theodosiou <ypercube@gmail.com> wrote:

On Fri, Jan 22, 2021 at 12:57 PM PG Bug reporting form <noreply@postgresql.org> wrote:

The following bug has been logged on the website:

Bug reference:      16834
Logged by:          Flavio Ricci
Email address:      flavio.ricci82@gmail.com
PostgreSQL version: 11.9
Operating system:   x86_64-pc-linux-gnu
Description:       

Hi,

I have found out that in  a where-in query if the nested query fails, the
parent query returns all the records if the where condition field matches
with the selected field in the inner query.
Example:
SELECT *
FROM  table_a
WHERE field_only_in_parent_table IN (
   SELECT field_only_in_parent_table
   FROM table_b
   WHERE name = 'John Doe')

If you run only the nested query it fails because the field does not exist
for table_b
If you run all the query, it returns all the records of table_a

Regards,

Flavio

This is not a bug. Returning all records of the parent table is the correct result, due to scope resolution.

The column field_only_in_parent_table does not have a table prefix so it is first checked whether it's a column of the immediate tables (table_b here). Since the column is not there, the next level tables are checked (table_a here). It is a there so the query is executed as:

SELECT *
FROM  table_a
WHERE field_only_in_parent_table IN (
   SELECT table_a.field_only_in_parent_table
   FROM table_b
   WHERE name = 'John Doe')

To be accurate the rows of table a that are returned are the ones wit non-null values in field_only_in_parent_table and only if table b has at least one row.
If table b has 0 rows, then the query returns no rows.