Re: [HACKERS] Odd subselect in target list behavior WRT aggregates

[Mike Mascari]

Tom Lane wrote:

"Mike Mascari" <[EMAIL PROTECTED]> 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
[EMAIL PROTECTED]




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