> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I thought the aggregate would be generated on all rows in the table in
> > the pre-transaction version of the table, so in this example:
> > regression=# update t2 set f2 = min(f1) from t1;
> > It places the minimum value of t1.f1 in all t2.f2 rows.
>
> This actually is not the most interesting example, because the aggregate
> doesn't depend at all on t2. Try this instead:
>
> regression=# create table t1(f1 int);
> CREATE
> regression=# create table t2(f1 int);
> CREATE
> regression=# insert into t1 values(-1);
> INSERT 400599 1
> regression=# insert into t1 values(-2);
> INSERT 400600 1
> regression=# insert into t1 values(-3);
> INSERT 400601 1
> regression=# insert into t2 values(-1);
> INSERT 400602 1
> regression=# insert into t2 values(-2);
> INSERT 400603 1
> regression=# insert into t2 values(-3);
> INSERT 400604 1
> regression=# update t2 set f1 = count(*) from t1;
> UPDATE 1
> regression=# select * from t2;
> f1
> ----
> -2
> -3
> 9
> (3 rows)
>
> regression=#
>
> This is certainly broken, but what's the correct behavior?
Shouldn't it be 9 because there is no join of t1 and t2?
I can also see 3 as a valid answer.
> Or how about this, which doesn't even use an aggregate:
>
> regression=# update t2 set f1 = t1.f1 from t1;
> UPDATE 3
> regression=# select * from t2;
> f1
> ----
> -1
> -1
> -1
> (3 rows)
>
> regression=#
>
> That's surprising too, perhaps, but what would you have expected
> and why?
So it grabs the first match. Seems reasonable because t1 returns more
than one row.
>
> There's a reason why SQL99 forbids joins and aggregates in UPDATE ...
> they're not always well-defined.
Yes, I see that now.
> I had a proposal (GROUP BY ctid) in the older thread for fixing the
> aggregate misbehavior, but it doesn't solve the more general problem
> of a join that produces multiple matches for the same target row.
> Seems like that probably ought to draw an error.
Or a NOTICE stating a random row was chosen.
-- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610)
853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill,
Pennsylvania19026