pete@phillipsfamily.freeserve.co.uk writes:
> select distinct year,month,
> (select sum(monthcustomer.number_of_items) from monthcustomer where
> monthcustomer.year=m.year and monthcustomer.month=m.month) as NumPots
> from monthcustomer m;
> This goes off and never comes back -
No surprise, considering the sub-select is going to be evaluated
separately for every row of monthcustomer --- and then most of those
evaluations will be thrown away by the DISTINCT :-(
A straightforward way of reducing the redundant computations would be
to do the DISTINCT first:
select year,month, (select sum(monthcustomer.number_of_items) from monthcustomer where monthcustomer.year=m.year and
monthcustomer.month=m.month)as NumPotsfrom (select distinct year, month from monthcustomer) as m;
But it appears to me that you are reinventing the wheel. Isn't this
query the equivalent of a grouped aggregation --- viz,
select year, month, sum(number_of_items) as NumPotsfrom monthcustomergroup by year, month
regards, tom lane
