Re: Altering a column type - Most efficient way

Ow Mun Heng schrieb:
> Is there any quick hacks to do this quickly? There's around
> 20-30million
> rows of data.
>
> I want to change a column type from varchar(4) to varchar()
>
> table size is ~10-15GB (and another 10-15G for indexes)
>
> What would be the preferrred way of doing it? SHould I be dropping the
> indexes 1st to make things faster? Would it matter?
>
> The punch line is that since the databases are connected via slony, this
> makes it even harder to pull it off. My last try got the DDL change
> completed in like 3 hours (smallest table of the bunch) and it hung
> everything
>
Before Postgresql supported "alter table ... type ... " conversions, I
did it a few times when I detected later that my varchar() fields were
too short, and it worked perfectly.

Example:
{OLDLEN} = 4
{NEWLEN} = 60

update pg_attribute
set atttypmod={NEWLEN}+4
where attname='the-name-of-the-column'
and attrelid=(select oid from pg_class where
relname='the-name-of-the-table')
and atttypmod={OLDLEN}+4;

This worked very well when you want to increase the maximum length,
don't try to reduce the maximum length this way!

Disclaimer: I do not know if slony might be have a problem with this.