Re: logical column position

Rod Taylor <pg(at)rbt(dot)ca> writes:
> On Mon, 2003-11-17 at 20:24, Christopher Kings-Lynne wrote:
>> BTW, one main consideration is that all the postgres admin apps will now
>> need to support ORDER BY attlognum for 7.5+.

> Yeah... how about maintaining attnum for the logical attribute number
> and create an attphysnum or something for the physical position instead?
> This is more intrusive into the source, but you don't need to teach new
> tricks to external entities.
> [ and similar remarks from other people elsewhere in the thread ]

It's completely fallacious to imagine that we could make this change be
transparent to external applications. To take two examples:

1. How many places do you think know that pg_attribute.attnum links to
pg_attrdef.adnum? pg_dump, psql, and the JDBC driver all appear to
know that, in a quick search of the CVS tree; I haven't even bothered to
look at pgadmin and the other apps that are likely to have such
dependencies.

2. How about linking pg_attribute.attnum to entries in pg_index.indkey?
Lots of apps know about that too.

Unless you are going to change the meanings of pg_index.indkey and
pg_attrdef.adnum, you can't simply redefine attnum as a logical column
position. And if you do make such a change you will break code
elsewhere.

If we add a *new* column attlogpos, without changing the semantics
of attnum, then I think we have a fighting chance of making this work
without an undue amount of effort. I see no prospect that we can
change the meaning of attnum without breaking things far and wide.

regards, tom lane