CAST doesn't work :-(

Hi.

I read in Momjian's book that to change the CHAR length of a column in a table (from 30 -> 40) lets say, i'm to use the
CASTcommand (pg 93). The fact is that the Posgres User's manual has nothing about cast and psql doesn't understand this
command:-((((((( 

Anyone can tell me how to ajust a CHAR length in a table column? I found out that some email exceed 30 digits now i
can'tinsert them and don't want to start from scratch. 

Thanks.

--
                 Antonio Gennarini
                Crecchio, CH - ITALY
    -----------------------------------
Earth Science student at University "G. D'Annunzio".
     Join th GNU generation, start from LINUX.