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;