I don't think this is the right approach. Creating a subtransaction for each row will cause substantial performance issues.
Subtransactions aren't created for each row. The block of rows in one subtransaction is 1000 (SAFE_BUFFER_SIZE) and can be changed. There is also a constraint for the number of bytes MAX_SAFE_BUFFER_BYTES in safe_buffer:
while (sfcstate->saved_tuples < SAFE_BUFFER_SIZE && sfcstate->safeBufferBytes < MAX_SAFE_BUFFER_BYTES)
We now can call data type input functions without throwing errors, see InputFunctionCallSafe(). Use that to avoid throwing an error instead of catching it.
InputFunctionCallSafe() is good for detecting errors from input-functions but there are such errors from NextCopyFrom () that can not be detected with InputFunctionCallSafe(), e.g. "wrong number of columns in row''. Do you offer to process input-function errors separately from other errors? Now all errors are processed in one "switch" loop in PG_CATCH, so this change can complicate code.
