Is it safe to increase pg_attribute.atttypmod ?

Hi-

We need to increase the length of a varchar field, and couldn't find a way
to do it using "alter table". Since the alternative is copying & re-creating
a very large table, we starting poking around under the hood & found that
pg_attribute.atttypmod seems to be always equal to the length plus 4 in the
case of varchar fields.

We created a test table and altered the varchar length on a field by
increasing pg_attribute.atttypmod, and the length seemed to change and act
as expected.

The update to change a varchar(10) field to varchar(40) looked like this:

update pg_attribute set atttypmod = 44
where attrelid = ( select oid from pg_class where relname = 'test' )
and attname = 'oldtest' ;

Is this a smart thing to do?

Are there other hidden related bits of data that will come back to haunt us
later?

-Nick

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