Re: ALTER TABLE DROP COLUMN

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

Folks, we had better choose one and get started.

Number 1 Hiroshi has ifdef'ed out in the code. Items 1 and 2 have
problems with backend code and 3rd party code not seeing the dropped
columns, or having gaps in the attno numbering. Number 3 has problems
with making it an atomic operation, and number 4 is described below.

---------------------------------------------------------------------------

> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > You can exclusively lock the table, then do a heap_getnext() scan over
> > the entire table, remove the dropped column, do a heap_insert(), then a
> > heap_delete() on the current tuple, making sure to skip over the tuples
> > inserted by the current transaction. When completed, remove the column
> > from pg_attribute, mark the transaction as committed (if desired), and
> > run vacuum over the table to remove the deleted rows.
>
> Hmm, that would work --- the new tuples commit at the same instant that
> the schema updates commit, so it should be correct. You have the 2x
> disk usage problem, but there's no way around that without losing
> rollback ability.
>
> A potentially tricky bit will be persuading the tuple-reading and tuple-
> writing subroutines to pay attention to different versions of the tuple
> structure for the same table. I haven't looked to see if this will be
> difficult or not. If you can pass the TupleDesc explicitly then it
> shouldn't be a problem.
>
> I'd suggest that the cleanup vacuum *not* be an automatic part of
> the operation; just recommend that people do it ASAP after dropping
> a column. Consider needing to drop several columns...
>
> regards, tom lane
>
> ************
>

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