Forbid FOR PORTION OF with WHERE CURRENT OF

Forbid FOR PORTION OF with WHERE CURRENT OF

It is not clear how the implicit condition of FOR PORTION OF should
interact with the use of a cursor.  Normally, we forbid combining
WHERE CURRENT OF with other WHERE conditions.  The SQL standard only
includes FOR PORTION OF with <update statement: searched> and <delete
statement: searched>, not <update statement: positioned> or <delete
statement: positioned>, so it is easy for us to exclude the
functionality, at least for now.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/flat/CA%2BrenyUEKPexUYsH4qeU8_o1jqKsUkEWca1keS6n21shgG1g%2BA%40mail.gmail.com