Re: Minor buglet in update...from (I think)

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Oh, so it is the aggregate.  What threw me off is that both parts of the
> > WHERE clause are required to cause the failure,
> 
> Not necessarily; I think it's got more to do with a null aggregate
> result:
> 
> regression=# create table t1 (f1 datetime);
> CREATE
> regression=# create table t2 (f2 datetime);
> CREATE
> regression=# update t2 set f2 = min(f1) from t1;
> ERROR:  ExecutePlan: (junk) `ctid' is NULL!
> regression=# insert into t1 values ('now');
> INSERT 400577 1
> regression=# update t2 set f2 = min(f1) from t1;
> ERROR:  ExecutePlan: (junk) `ctid' is NULL!
> regression=# insert into t2 values ('now');
> INSERT 400578 1
> regression=# update t2 set f2 = min(f1) from t1;
> UPDATE 1
> regression=#
> 
> However the ERROR is only one symptom.  The real problem is that the
> calculation that's being done is useless/nonsensical.
> 
> > I don't see a problem with aggregates in UPDATE,
> 
> Think harder ... what is the aggregate being taken over, and how do you
> associate the aggregate's single result row with any particular row in
> the UPDATE's target table?

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.  Is there
another way to look at it?

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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