Re: BUG #4689: Expanding the length of a VARCHAR column should not induce a table rewrite - Mailing list pgsql-bugs

Heikki Linnakangas wrote:
> xuan--2009.03--submitbug--support--postgresql.org@baldauf.org wrote:
>> When executing
>> "ALTER TABLE sometable ALTER COLUMN somecolumn TYPE VARCHAR(7)", the
>> whole
>> table is re-written, and this rewrite takes many hours. During these
>> hours,
>> all writers on this table stall, making the database effectively
>> unavailable.
>>
>> However, in almost all cases, there is no need for such relaxing of
>> limits
>> to require a table rewrite.
>
> While this isn't a bug, it's a reasonable feature request. I've added
> this to the TODO page: http://wiki.postgresql.org/wiki/Todo#ALTER
>
> Patches are welcome.

The question is how you want to implement this in a data type
independent fashion.  You can't assume that increasing the typmod is a
noop for all data types.