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Thread: Document behaviour of failed sub queries

Document behaviour of failed sub queries

From
PG Doc comments form
Date:
The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/functions-subquery.html
Description:

I have found that when a sub query fails, the preceeding IN will ignore the
failure and execute as if the subquery did not exist.

I think this should either be stated explicitly in the documentation or it
should fail the main the query.

For example:
UPDATE table1 SET status='expired' WHERE id in (SELECT wrong_id IN table2)

This will update every row in table1if wrong_id doesn't exist, ignoring the
ERROR:  column "wrong_id" does not exist from the subquery.

If you feel like the documentation explains this thoroughly enough already
then please point me in the direction. At the moment it makes refernece to
queries not completing but it doesn't mention failures.

Thanks

Jack

Re: Document behaviour of failed sub queries

From
"David G. Johnston"
Date:
On Mon, Jun 28, 2021 at 8:34 AM PG Doc comments form <noreply@postgresql.org> wrote:
For example:
UPDATE table1 SET status='expired' WHERE id in (SELECT wrong_id IN table2)

This will update every row in table1if wrong_id doesn't exist, ignoring the
ERROR:  column "wrong_id" does not exist from the subquery.

The subquery never provokes that error by virtue of the fact it is a subquery.  It's only if you run that as a standalone query do you see the error.  This is because correlated subqueries are a thing (and, yes, they are documented).

https://wiki.postgresql.org/wiki/FAQ#Why_doesn.27t_PostgreSQL_report_a_column_not_found_error_when_using_the_wrong_name_in_a_subquery.3F

David J.

Re: Document behaviour of failed sub queries

From
"David G. Johnston"
Date:
On Mon, Jun 28, 2021 at 8:40 AM David G. Johnston <david.g.johnston@gmail.com> wrote:
On Mon, Jun 28, 2021 at 8:34 AM PG Doc comments form <noreply@postgresql.org> wrote:
For example:
UPDATE table1 SET status='expired' WHERE id in (SELECT wrong_id IN table2)

This will update every row in table1if wrong_id doesn't exist, ignoring the
ERROR:  column "wrong_id" does not exist from the subquery.

The subquery never provokes that error by virtue of the fact it is a subquery.  It's only if you run that as a standalone query do you see the error.  This is because correlated subqueries are a thing (and, yes, they are documented).



I may have mis-read your email...the behavior I describe is usually what prompts these kinds of questions but your example doesn't actually fit the pattern.  I find it hard to believe that what you describe is really happening...though usually with IN clauses the presence of NULLs can confound things.

You should put together a self-contained reproducer script and post it as a bug report once you've confirmed it produces the problem you describe while using psql and a current version of PostgreSQL.

David J.