Altering a column (increasing size) in Postgres takes long time

We have a huge table with hundred million records. We need to increase
the size of an existing column with varchar type, and it has been
running for more than 12 hours.


We're using: ALTER TABLE Property ALTER COLUMN "situs-number" TYPE
varchar(30);


The index on that column has been dropped before issuing the above
statement.


1)       Can you explain what's happening internally that make this a
very long process? Does the table get re-created?


2)       Assuming the Alter statement finished successfully, And if I
didn't drop the index (on that column), do I have to rebuild the index?
Does the index get invalidated for just alter the indexed column?


3)       Some folks referred to directly updating Postgres internal
tables (pg_attribute) which takes seconds to make the column change
happen. How safe if this and would potentially cause any corruption?


SET SESSION AUTHORIZATION 'postgres';


BEGIN;


update pg_attribute

set atttypmod = 21 + 4

where attrelid = 'property'::regclass

and attname = 'situs-number';


update pg_attribute

set atttypmod = 21 + 4

where attrelid = 'interim-refresh'::regclass

and attname = 'situs-number';


update pg_attribute

set atttypmod = 21 + 4

where attrelid = 'interim-drefresh'::regclass

and attname = 'situs-number';


update pg_attribute

set atttypmod = 21 + 4

where attrelid = 'interim-update-property'::regclass

and attname = 'situs-number';


update pg_attribute

set atttypmod = 21 + 4

where attrelid = 'ix-property-address'::regclass

and attname = 'situs-number';


RESET SESSION AUTHORIZATION;



4)       Is there a more practical and safe method  to alter a huge
table with reasonable amount of time?


Please advise. Your help is much appreciated.

We're running Postgres 8.3.7 on RedHat Enterprise AS 4.7 on HP585DL.


Regards,
     Husam

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