Andreas Pflug <[EMAIL PROTECTED]> writes:In the mind of a programmer, a ALTER COLUMN doesn't create a new column, but merely changes some attributes of an existing column. In this sense, changeability and immutability are not controversal.
Maybe my proposal wasn't clear enough:
Just as an index references a pg_class entry by it's OID, not some value identifying it's physical storage, all objects might continue referencing columns by attnum.
That's exactly the same thing I am saying. Your mistake is to assume
that this function can be combined with identification of a (changeable)
logical column position. It can't. Changeability and immutability are
just not compatible requirements.
Digging deeper:
TupDesc contains an array of physical attr descriptions, and to access a column description attnum is taken as index into that array (taken from fastgetattr).
return fetchatt(tupleDesc->attrs[attnum-1], ...)
The physical location can easily reordered if there's an additional array, to translate attnum into the array index.
return fetchatt(tupleDesc->attrs[tupleDesc->attrpos[attnum-1]] ...
For sure, reordering (i.e. changing the attrpos array) may only be performed as long as the column isn't referenced.
Regards, Andreas
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