BUG #18307: system columns does not support using join

On Tuesday, January 23, 2024, PG Bug reporting form <noreply@postgresql.org>
wrote:

The following bug has been logged on the website:

Bug reference: 18307
Logged by: RekGRpth
Email address: rekgrpth@gmail.com
PostgreSQL version: 16.1
Operating system: docker alpine
Description:

create table t(i int);

explain (costs off) select * from t join t tt on t.xmin = tt.xmin;
QUERY PLAN
---------------------------------
Hash Join
Hash Cond: (t.xmin = tt.xmin)
-> Seq Scan on t
-> Hash
-> Seq Scan on t tt
(5 rows)

explain (costs off) select * from t join t tt using (xmin);
ERROR: column "xmin" specified in USING clause does not exist in left
table

I don’t this being worth the effort to change, and really seems like
completely expected behavior. “Select *” doesn’t output xmin, it requires
explicit table qualification to see it. This is the same thing.

David J.

Thanks, I'll look into it.

explain (costs off) select t.xmin from t join t tt on t.xmin = tt.xmin;
QUERY PLAN
---------------------------------
Hash Join
Hash Cond: (t.xmin = tt.xmin)
-> Seq Scan on t
-> Hash
-> Seq Scan on t tt
(5 rows)

explain (costs off) select t.xmin from t join t tt using (xmin);
ERROR: column "xmin" specified in USING clause does not exist in left table

explain (costs off) select tt.xmin from t join t tt using (xmin);
ERROR: column "xmin" specified in USING clause does not exist in left table

explain (costs off) select t.xmin, tt.xmin from t join t tt using (xmin);
ERROR: column "xmin" specified in USING clause does not exist in left table

ср, 24 янв. 2024 г. в 19:26, David G. Johnston <david.g.johnston@gmail.com>:

"David G. Johnston" <david.g.johnston@gmail.com> writes:

On Tuesday, January 23, 2024, PG Bug reporting form <noreply@postgresql.org>
wrote:

create table t(i int);

explain (costs off) select * from t join t tt on t.xmin = tt.xmin;
QUERY PLAN
---------------------------------
Hash Join
Hash Cond: (t.xmin = tt.xmin)
-> Seq Scan on t
-> Hash
-> Seq Scan on t tt
(5 rows)

explain (costs off) select * from t join t tt using (xmin);
ERROR: column "xmin" specified in USING clause does not exist in left
table

I don’t this being worth the effort to change, and really seems like
completely expected behavior. “Select *” doesn’t output xmin, it requires
explicit table qualification to see it. This is the same thing.

Well, it is odd that "using (xmin)" isn't equivalent to the allegedly
equivalent "on t.xmin = tt.xmin". This is down to the infrastructure
in transformFromClauseItem(), which searches the lists of (regular,
non-system) relation output column names to expand USING(). But like
you, I can't get excited about changing it. There are a couple of
practical reasons why not:

* NATURAL JOIN is defined in terms of USING. But we *certainly* don't
want "x NATURAL JOIN y" deciding that it should equate all the system
columns of x to those of y. So there's going to be inconsistency at
one level or the other no matter what.

* I really find it hard to imagine a valid use case for joining on any
system column. There are use-cases for joining on TID in an UPDATE
involving a self-join to the target table; but you can't write that
with JOIN USING syntax.

regards, tom lane