Comment 1966 added to page ddl-alter.html of version 7.4 - Mailing list pgsql-www

Author: Alex French <alexfrench@gmail.com>
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Note that pg does not support changing the type of a column. Instead, use something like the code below to change, say,
avarchar(32) into varchar(64) (assuming the collumn is called abc): 

BEGIN;
ALTER TABLE mytable ADD COLUMN abc_new varchar(64);
UPDATE mytable SET abc_new = abc;
ALTER TABLE mytable RENAME abc TO abc_old;
ALTER TABLE mytable RENAME abc_new TO abc;
COMMIT;

If you're chaning the type of the column radically, you need to do more fancy type casting, but that's the theory.
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Manual page: http://wwwmaster.postgresql.org/docs/7.4/interactive/ddl-alter.html
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