Re: column label

> >> Ok I have a query in a program that looks something like this
> >>
> >> select
> >> reference_number as foo,
> >> '(10,10)'::point <-> point(x,y) as distance
> >> from imagemap
> >> where
> >> index_number =3D '2322' and
> >> block_number =3D '140'
> >> order by distance
> >> limit 1;
> >>
> >> I wanted to add a clause in where 'and distance < 30'
> >> but according to the postgres manual it says I cannot do that for
> >> WHERE or HAVING clauses, instead use the real name.
> >>
> >> However dropping the 'as distance' from my query gives me a column
> >> name of '?column?' which happens to not be worth a dang thing.
> >>
> >> How can it be done?
> >>
> >> (I suddenly have a feeling its going to require a subselect)
> > Your feeling is 50% accurate. If you want to use subselect, throw
"<10"
> > into this subselect, because you will have performance loss.
> >
> > You can also use whole function in where clause and order by:
> > select
> > reference_number as foo,
> > '(10,10)'::point <-> point(x,y) as distance
> > from imagemap
> > where
> > index_number =3D '2322' and
> > block_number =3D '140'
> > where '(10,10)'::point <-> point(x,y)<10
> > order by '(10,10)'::point <-> point(x,y)
> > limit 1;
> >
> Thats kind of a pain... I'd rather perform the calculation once and
then
> do my order and where on the one value. Is there any reason why
postgres
> does not recognize labels under WHERE?
>
So how about
select foo, distance from (
select
reference_number as foo,
'(10,10)'::point <-> point(x,y) as distance
from imagemap
where
index_number = '2322' and
block_number = '140'
) as bar
where distance < 30
order by distance
limit 1;
The reason why postgres does not recognize labels under WHERE
is the WHERE clause is evaluated before column aliasing takes place.
But there is this valuable feature of using a sub-SELECT as
from_item, so this should not be a problem at all.
Alternatively you could use

select
reference_number,
'(10,10)'::point <-> point(x,y)
from imagemap alias (foo, distance)
where
index_number = '2322' and
block_number = '140' and distance < 30
order by distance
limit 1;

Does this help?

Regards, Christoph