Re: [HACKERS] Re: Join/table alias bug

Peter Eisentraut <e99re41@DoCS.UU.SE> writes:
> ... The reason this behaves that way is because queries
> without from lists (SELECT my_tbl.a) are valid in PostgreSQL for
> historical reasons, so we're stuck with it.

Not only for historical reasons: there are cases where it allows you
to do things you couldn't easily do otherwise.  An example is deleting
using a join:

    DELETE FROM target WHERE field1 = source.field2

which deletes any record in target whose field1 matches any field2
value in source.  This isn't SQL92 since DELETE doesn't allow you
to specify any tables except the target table in FROM.  (Yeah,
I know this example could be written with a subselect --- but with
a more complex WHERE condition it gets harder to do that.  Also
slower.)

> We've pondered many times about emitting warnings but a definite
> consensus was never reached.

Bruce had actually put in some code to emit warnings, but Thomas
objected to it for reasons I don't recall clearly.  I think it was
an implementation issue rather than objecting to the idea of having
warnings.  AFAIR we had pretty much agreed that a warning would be
a good idea.

IIRC, Bruce's code would emit a warning whenever an implicit RTE was
added.  I think that might be overly verbose --- I'd be inclined to
warn only in the case that an implicit RTE is added for a table that
has an RTE already (under a different alias).  That is the only
situation I've seen user complaints about.

            regards, tom lane