Обсуждение: Odd subselect in target list behavior WRT aggregates

Hello. I have some code which generates subselects in the target
list of a query and then aggregates the results. The code allows
the user to determine the attributes of the aggregation. If the
user chooses to aggregate on the same value twice, I get the
"Sub-SELECT" error. If the user chooses a different second
attribute of aggregation, no error occurs. Is that correct
behavior? The only difference between Query #1 and Query #2 is
that the second subselect in the target list of Query #2
aggregates on the 'day' of a sale as opposed to the 'hour':

Query #1
--------

SELECT SUM(p.dstqty) as agg,
(SELECT date_trunc('hour', sales.active)FROM salesWHERE p.purchase = sales.purchase) as field1,
(SELECT date_trunc('hour', sales.active)FROM salesWHERE p.purchase = sales.purchase) as field2

FROM purchases p
WHERE ...
GROUP BY 2,3;

ERROR:  Sub-SELECT uses un-GROUPed attribute p.purchase from
outer query

Query #2
--------

SELECT SUM(p.dstqty) as agg,
(SELECT date_trunc('hour', sales.active)FROM salesWHERE p.purchase = sales.purchase) as field1,
(SELECT date_trunc('day', sales.active)FROM sales WHERE p.purchase = sales.purchase) as field2

FROM purchases p
WHERE ...
GROUP BY 2,3;
agg   |         field1         |         field2
-------+------------------------+------------------------
1.0000 | 2002-12-27 18:00:00-05 | 2002-12-27 00:00:00-05

I also failed to mention in the original post that this is
PostgreSQL version 7.2.1.

Any help or instruction would be greatly appreciated.

Mike Mascari
mascarm@mascari.com

"Mike Mascari" <mascarm@mascari.com> writes:
> Hello. I have some code which generates subselects in the target
> list of a query and then aggregates the results. The code allows
> the user to determine the attributes of the aggregation. If the
> user chooses to aggregate on the same value twice, I get the
> "Sub-SELECT" error. If the user chooses a different second
> attribute of aggregation, no error occurs. Is that correct
> behavior?

This seems to be fixed as of 7.3, though I do not recall a previous
bug report like it.

However, I wonder why you are doing it like that, and not with a join:

SELECT SUM(p.dstqty) as agg,      date_trunc('hour', sales.active) as field1,      date_trunc('day', sales.active) as
field2
FROM purchases p, sales
WHERE p.purchase = sales.purchase     AND ...
GROUP BY 2,3;

The multiple-sub-select approach will require a separate probe into
"sales" to retrieve each of the fields; there's no optimization across
different subselects.
        regards, tom lane

Tom Lane wrote:
> "Mike Mascari" <mascarm@mascari.com> writes:
> 
>>Hello. I have some code which generates subselects in the target
>>list of a query and then aggregates the results. The code allows
>>the user to determine the attributes of the aggregation. If the
>>user chooses to aggregate on the same value twice, I get the
>>"Sub-SELECT" error. If the user chooses a different second
>>attribute of aggregation, no error occurs. Is that correct
>>behavior?
> 
> 
> This seems to be fixed as of 7.3, though I do not recall a previous
> bug report like it.

Thanks, Tom. I should have tried the current version before posting.

> 
> However, I wonder why you are doing it like that, and not with a join:
> 
> SELECT SUM(p.dstqty) as agg,
>        date_trunc('hour', sales.active) as field1,
>        date_trunc('day', sales.active) as field2
> FROM purchases p, sales
> WHERE p.purchase = sales.purchase
>       AND ...
> GROUP BY 2,3;
> 
> The multiple-sub-select approach will require a separate probe into
> "sales" to retrieve each of the fields; there's no optimization across
> different subselects.

Normally, the grouping is done on two or more distantly related 
pieces of data:

"How many widgets were sold by John on Mondays?"
"What is the most popular hour for sales by quarter?"

etc.

So the nature of the data is such that to dynamically generate 
the proper joins in the FROM/WHERE clause was too complex (for 
me). :-)

Thanks again,

Mike Mascari
mascarm@mascari.com