Re: [HACKERS] ALTER TABLE DROP COLUMN

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