Обсуждение: Geometric types in SELECT?

  First off, thanks for the answer to my earlier question -- I've
become less misguided since (I hope!)
  One of my tables uses the point data type.  I wanted to use this to
find points close to one another.  Unfortunately, when I do something
like:

=>select * from address order by length ( '((0,0), coord)'::lseg );

or

=>select length( '((0,0),coord)'::lseg ) from address;

I receive this error:

Bad lseg external representation '((0,0), coord)'

If I replace coord with an actual value, I don't get the error. Am I
somehow mangling the syntax?  I've tried a number of different formats
for the query, with the same error each time.  Is it possible to do this?

Thanks,

-mike

-- 

And though the holes were rather small
They had to count them all
Now they know how many holes it takes to fill the Albert Hall
*I'd love to turn you on*
 -- Beatles, "A Day in the Life"

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Key ID = 1024D/9A256AE5 1999-11-13 Mike Glover <mpg4@duluoz.net>
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mpg4@duluoz.net writes:
> =>select length( '((0,0),coord)'::lseg ) from address;

> I receive this error:

> Bad lseg external representation '((0,0), coord)'

> If I replace coord with an actual value, I don't get the error. Am I
> somehow mangling the syntax?

Yup.  '((0,0),coord)'::lseg is a literal constant --- in general,
'anything'::typename is a literal constant in Postgres.  And it's
not a valid literal constant for lseg.

I suppose coord is the name of a point column in your table?  To do it
correctly, you'd need something like
select length( makelseg( '(0,0)'::point, coord )) from table;

I'm not sure if there is a function that makes an lseg from two points,
and if there is it probably isn't called "makelseg", but hopefully you
get the idea.

I am pretty sure there is a function that computes the distance between
two points, so what you are likely to end up really writing is
select distance( '(0,0)'::point, coord ) from table;

(after you dig through the documentation to find the actual name
of that function...)
        regards, tom lane