"Christopher Kings-Lynne" <[EMAIL PROTECTED]> 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

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