Odd Update Behaviour

Hi,

I noticed some weird update behaviour today in one of our development
environments. In my opinion this appears to be a bug, but before reporting
it I thought I should seek the opinions of others in the community. Maybe
this is known and/or seen before?

The behaviour is visible when a subquery is used in an update. More
specifically when a subquery is rejected by the parser when executed on
it's own. This causes the where condition to be ignored entirely and thus
the whole table updated rather than the parser throwing an error.

Example below that can be use to replicate:
update_bug=# create table dummy_data (rowid bigserial primary key, type
int);
CREATE TABLE

update_bug=# insert into dummy_data (type) select 1 from
generate_series(1,10000);
INSERT 0 10000

update_bug=# insert into dummy_data (type) select 2 from
generate_series(1,10000);
INSERT 0 10000

update_bug=# select type, count(*) from dummy_data group by 1 order by 1;
type | count
------+-------
1 | 10000
2 | 10000
(2 rows)

update_bug=# select rowid from (select rowid as idnumber from dummy_data
where type = 1) q;
ERROR: column "rowid" does not exist
LINE 1: select rowid from (select rowid as idnumber from dummy_data ...

update_bug=# update dummy_data set type = 3 where rowid in (select rowid
from (select rowid as idnumber from dummy_data where type = 1) q);
UPDATE 20000

update_bug=# select type, count(*) from dummy_data group by 1 order by 1;
type | count
------+-------
3 | 20000
(1 row)

I have managed to replicate this behaviour on both 9.5.16 & 10.6 and will
continue to test further.

Any thoughts/opinions are obviously welcomed.

Best wishes,
Harry