Thread: column label

Ok I have a query in a program that looks something like this

selectreference_number as foo,'(10,10)'::point <-> point(x,y) as distance
from imagemapwhere index_number = '2322' and block_number = '140'order by distancelimit 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)

Ryan

Uz.ytkownik Ryan napisa?:
> 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 = '2322' and
>   block_number = '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 = '2322' and   block_number = '140'  where '(10,10)'::point
<->point(x,y)<10  order by '(10,10)'::point <-> point(x,y)  limit 1; 

BTW Did you try to use polygons in your image maps, or you have rather
point oriented web page than area oriented?

Regards,
Tomasz Myrta

> Uz.ytkownik Ryan napisa?:
>> 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 = '2322' and
>>   block_number = '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 = '2322' and
>     block_number = '140'
>    where '(10,10)'::point <-> point(x,y)<10
>    order by '(10,10)'::point <-> point(x,y)
>    limit 1;
>
> BTW Did you try to use polygons in your image maps, or you have rather
> point oriented web page than area oriented?
>
> Regards,
> Tomasz Myrta
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?

The reason I use points is because my customized web-based image
mapping/ocr program can deal with points very aptly.  Also with a distance
based calc overlap is not really an issue as with polygons.  The reason I
am trying to limit the distance is simply to cut out clicking on far away
parts of the image.  Heck, I could check distance on the program side of
things but I try and keep as much in SQL as possible.

Ryan

> >> 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 distancefrom 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
selectreference_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
distancelimit 1;
 

Does this help?

Regards, Christoph