a315b967cc
This is expected to be useful mostly when performing such scans in parallel, because in that case it allows (in combination with commit acf555bc53acb589b5a2827e65d655fa8c9adee0) nodes below a Gather to get control just before the DSM segment goes away. KaiGai Kohei, except that I rewrote the documentation. Reviewed by Claudio Freire. Discussion: http://postgr.es/m/CADyhKSXJK0jUJ8rWv4AmKDhsUh124_rEn39eqgfC5D8fu6xVuw@mail.gmail.com