Don't generate parallel paths for rels with parallel-restricted outputs.

Such paths are unsafe.  To make it cheaper to detect when this case
applies, track whether a relation's default PathTarget contains any
non-Vars.  In most cases, the answer will be no, which enables us to
determine cheaply that the target list for a proposed path is
parallel-safe.  However, subquery pull-up can create cases that
require us to inspect the target list more carefully.

Amit Kapila, reviewed by me.

src/backend/optimizer/util/placeholder.c

@@ -393,6 +393,7 @@ add_placeholders_to_base_rels(PlannerInfo *root)
 
 			rel->reltarget->exprs = lappend(rel->reltarget->exprs,
 											copyObject(phinfo->ph_var));
+			rel->reltarget_has_non_vars = true;
 			/* reltarget's cost and width fields will be updated later */
 		}
 	}
@@ -427,6 +428,7 @@ add_placeholders_to_joinrel(PlannerInfo *root, RelOptInfo *joinrel,
 				/* Yup, add it to the output */
 				joinrel->reltarget->exprs = lappend(joinrel->reltarget->exprs,
 													phinfo->ph_var);
+				joinrel->reltarget_has_non_vars = true;
 				joinrel->reltarget->width += phinfo->ph_width;
 
 				/*