Re: ALTER TABLE DROP COLUMN

> > -----Original Message-----
> > From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
> >
> > >> Seems we have 4 DROP COLUMN ideas:
> > >> Method Advantage
> > >> -----------------------------------------------------------------
> > >> 1 invisible column marked by negative attnum fast
> > >> 2 invisible column marked by is_dropped column fast
> > >> 3 make copy of table without column col removed
> > >> 4 make new tuples in existing table without column col removed
> >
>
> Hmm,I've received no pg-ML mails for more than 1 day.
> What's happened with pgsql ML ?

Tom says there are tons of messages in the hub.org mail queue, but they
are not being delivered.

>
> > Bruce and I talked about this by phone yesterday, and we realized that
> > none of these are very satisfactory. #1 and #2 both have the flaw that
> > applications that examine pg_attribute will probably break: they will
> > see a sequence of attnum values with gaps in it. And what should the
> > rel's relnatts field be set to? #3 and #4 are better on that point,
> > but they leave us with the problem of renumbering references to columns
> > after the dropped one in constraints, rules, PL functions, etc.
> >
> > Furthermore, there is a closely related problem that none of these
> > approaches give us much help on: recursive ALTER TABLE ADD COLUMN.
> > Right now, ADD puts the new column at the end of each table it's added
> > to, which often means that it gets a different column number in child
> > tables than in parent tables. That leads to havoc for pg_dump.
> >
>
> Inheritance is one of the reason why I didn't take #2. I don't understand
> marking is_dropped is needed or not when pg_attribute is overhauled
> for inheritance.
> I myself have never wanted to use current inheritance functionality
> mainly because of this big flaw. Judging from the recent discussion
> about oo(though I don't understand details),the change seems to be
> needed in order to make inheritance functionality really useful.

What would happen is that all the logical attributes would be shifted
over, and a new column added using ADD COLUMN would be put in its place.
Seems it would work fine.

--
Bruce Momjian | http://www.op.net/~candle
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