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@candle.pha.pa.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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