Re: Adding CORRESPONDING to Set Operations

On Sun, Sep 18, 2011 at 5:39 AM, Kerem Kat <keremkat(at)gmail(dot)com> wrote:
> I am new to postgresql code, I would like to start implementing easyish TODO
> items. I have read most of the development guidelines, faqs, articles by
> Greg Smith (Hacking Postgres with UDFs, Adding WHEN to triggers).
> The item I would like to implement is adding CORRESPONDING [BY
> (col1[,col2,...]])] to INTERSECT and EXCEPT operators.
> Can anyone comment on how much effort this item needs?

This seems reasonably tricky for a first project, but maybe not out of
reach if you are a skilled C hacker. It's certainly more complicated
than my first patch:

http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=a0b76dc662efde6e02921c2d16e06418483b7534

I guess the first question that needs to be answered here is ... what
exactly is this syntax supposed to do? A little looking around
suggests that EXCEPT CORRESPONDING is supposed to make the
correspondence run by column names rather than by column positions,
and if you further add BY col1, ... then it restricts the comparison
to those columns.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company