Обсуждение: ALTER TABLE - ALTER COLUMN question

When I issue an alter table ..... alter column command, the table contents are copied from the old structure to the new, correct?  So I need 2x the table space available so the old and new tables can exist at the same time?  PostgreSQL doesn't use an in-place alter-table for these types of changes, correct? 

In this example the created and msg_datetime columns are currently defined as TIMESTAMP WITHOUT TIME ZONE and no default.  If the alter table for these changes is not in-place then I would want to run it as a single alter with all of the changes, such as this, instead of individual alters for each change, correct?  (Because each individual alter would require another table copy?)

ALTER TABLE my_table ALTER COLUMN created TYPE TIMESTAMP WITH TIME ZONE,
  ALTER COLUMN created SET DEFAULT   ('now'::text)::TIMESTAMP(6) WITH TIME ZONE,
  ALTER COLUMN msg_datetime TYPE TIMESTAMP WITH TIME ZONE,
  ALTER COLUMN msg_datetime SET DEFAULT ('now'::text)::TIMESTAMP(6) WITH TIME ZONE;

Thanks,

Keaton

Keaton Adams <kadams@mxlogic.com> writes:
> In this example the created and msg_datetime columns are currently
> defined as TIMESTAMP WITHOUT TIME ZONE and no default.  If the alter
> table for these changes is not in-place then I would want to run it as a
> single alter with all of the changes, such as this, instead of
> individual alters for each change, correct?  (Because each individual
> alter would require another table copy?)

That's why ALTER TABLE has the ability to do more than one thing in
parallel ...

            regards, tom lane

After I sent my request I found the notes related to ALTER TABLE on page 1,331 of the 8.1 manual, so that helped to explain it.  Your reply to my post confirmed what I thought I was reading.

Thanks for the reply,

Keaton



On Tue, 2007-05-29 at 17:16 -0400, Tom Lane wrote:

Keaton Adams <kadams@mxlogic.com> writes:
> In this example the created and msg_datetime columns are currently
> defined as TIMESTAMP WITHOUT TIME ZONE and no default.  If the alter
> table for these changes is not in-place then I would want to run it as a
> single alter with all of the changes, such as this, instead of
> individual alters for each change, correct?  (Because each individual
> alter would require another table copy?)

That's why ALTER TABLE has the ability to do more than one thing in
parallel ...

			regards, tom lane