Reduce "X = X" to "X IS NOT NULL", if it's easy to do so.

If the operator is a strict btree equality operator, and X isn't volatile,
then the clause must yield true for any non-null value of X, or null if X
is null.  At top level of a WHERE clause, we can ignore the distinction
between false and null results, so it's valid to simplify the clause to
"X IS NOT NULL".  This is a useful improvement mainly because we'll get
a far better selectivity estimate in most cases.

Because such cases seldom arise in well-written queries, it is unappetizing
to expend a lot of planner cycles looking for them ... but it turns out
that there's a place we can shoehorn this in practically for free, because
equivclass.c already has to detect and reject candidate equivalences of the
form X = X.  That doesn't catch every place that it would be valid to
simplify to X IS NOT NULL, but it catches the typical case.  Working harder
doesn't seem justified.

Patch by me, reviewed by Petr Jelinek

Discussion: https://postgr.es/m/CAMjNa7cC4X9YR-vAJS-jSYCajhRDvJQnN7m2sLH1wLh-_Z2bsw@mail.gmail.com

src/backend/optimizer/plan/initsplan.c

@@ -1964,10 +1964,11 @@ distribute_qual_to_rels(PlannerInfo *root, Node *clause,
 		if (maybe_equivalence)
 		{
 			if (check_equivalence_delay(root, restrictinfo) &&
-				process_equivalence(root, restrictinfo, below_outer_join))
+				process_equivalence(root, &restrictinfo, below_outer_join))
 				return;
 			/* EC rejected it, so set left_ec/right_ec the hard way ... */
-			initialize_mergeclause_eclasses(root, restrictinfo);
+			if (restrictinfo->mergeopfamilies)	/* EC might have changed this */
+				initialize_mergeclause_eclasses(root, restrictinfo);
 			/* ... and fall through to distribute_restrictinfo_to_rels */
 		}
 		else if (maybe_outer_join && restrictinfo->can_join)