"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
>> There might be other cases of legal direct change of system catalog
>> entries, e,g. varchar to text, if they all are only names for internally
>> identical data structures. Can you tell which datatypes may be legally
>> interchanged?
> Yes, you can check if they're binary compatible from the pg_cast table....
But nearly all of the interesting cases require you to understand the
type's interpretation of typmod, and you can't learn that from a table.
How many cases are there where blindly looking for a binary-compatible
cast in pg_cast will really do you much good? AFAICS you'd have to set
atttypmod to -1 if you change atttypid without knowing very specifically
what you are changing from and to.
regards, tom lane