Re: distance operator dont work

On Sat, Aug 12, 2006 at 08:52:48AM +0200, bernard snyers wrote:
> I have a table containing polygon and I try a distance operator which
> fails. (select * from mytable order by point(10,10) <-> mycol)
>
> Postgres responds that the operator <-> between a point and a polygon is
> unknown.
>
> I look in the pg_operator.h (in src/include/catalog) where I think all
> builtin operators are defined and found these definitions

Are you aware that you can query the system catalogs with SQL instead
of digging into header files? See also psql's \do command.

http://www.postgresql.org/docs/8.1/interactive/catalogs.html
http://www.postgresql.org/docs/8.1/interactive/catalog-pg-operator.html
http://www.postgresql.org/docs/8.1/interactive/app-psql.html

> and there is none between a point and a polygon. (600 604) (furthermore
> the 628 type is defined in ptypes.h as not implemented.
>
> I did the following test,
> I patch the line
> DATA(insert OID = 613 ( "<->" PGNSP PGUID b f 600 628 701
> 0 0 0 0 0 0 dist_pl - - ));
>
> replacing 628 by 604 ,
> compiling and reinstalling everything including the database

The dist_pl function expects a line argument; calling it with a
different type could cause unexpected results or even a server
crash. And are you aware that you can use CREATE OPERATOR instead
of hacking the source code and recompiling?

http://www.postgresql.org/docs/8.1/interactive/sql-createoperator.html

> and It works (I didnt verify the result)

In what sense does it work if you didn't verify the result?

> So I am wondering , if I miss something, do I have to convert my point
> in a degenerated circle or polygon) or do you forget one definition ?

I'd guess that certain features remain unimplemented due to lack
of interest; maybe one of the developers can comment on the history
of support for geometry operations. A nice thing about PostgreSQL
is that it's extensible without having to hack the source code: you
can write your own functions in PL/pgSQL or C or some other language
and create operators that call those functions. Such customizations
survive migrations via database dumps so you don't have to remember
to patch the code every time you upgrade.

If you're working with spatial data then you might want to check
out PostGIS:

http://www.postgis.org/

--
Michael Fuhr