per bug report from Daren Sefcik <daren@partnersdata.com>.

indeed some of 'em were missing support for more node types than that...

Add some internal cross references to select.sgml.

 Thanks to Goran Uddeborg for spotting this.

result relation ... wrong ...

a SubLink with the subplan's targetlist.  This fixes a problem seen with,
for example, a subselect that uses GROUP BY.

thereby guaranteeing wrong results from a rescanned Group node.

that contain null fields.  Old code would produce erratic sort results
because comparisons of tuples containing nulls could produce inconsistent
answers.