Just now, I tried to convert an integer column in a PostgreSQL database to one of type VARCHAR. I knew you had to do an explicit cast, so I was a bit stumped that I still wasn't allowed to perform the ALTER TABLE.
mydb=> ALTER TABLE mytable ALTER COLUMN mycolumn TYPE VARCHAR(31) USING mycolumn::text; ERROR: operator does not exist: character varying >= integer HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.
Wait what? A
mydb=> \d mytable ... Check constraints: "mytable_mycolumn_check" CHECK (mycolumn >= 0) ...
In the Netherlands we call it looking further than one's nose is long.
mydb=> ALTER TABLE DROP CONSTRAINT mytable_mycolumn_check; ALTER TABLE mydb=> ALTER TABLE mytable ALTER COLUMN mycolumn TYPE VARCHAR(31) USING mycolumn::text; ALTER TABLE