Add better handling of redundant IS [NOT] NULL quals

Until now PostgreSQL has not been very smart about optimizing away IS
NOT NULL base quals on columns defined as NOT NULL.  The evaluation of
these needless quals adds overhead.  Ordinarily, anyone who came
complaining about that would likely just have been told to not include
the qual in their query if it's not required.  However, a recent bug
report indicates this might not always be possible.

Bug 17540 highlighted that when we optimize Min/Max aggregates the IS NOT
NULL qual that the planner adds to make the rewritten plan ignore NULLs
can cause issues with poor index choice.  That particular case
demonstrated that other quals, especially ones where no statistics are
available to allow the planner a chance at estimating an approximate
selectivity for can result in poor index choice due to cheap startup paths
being prefered with LIMIT 1.

Here we take generic approach to fixing this by having the planner check
for NOT NULL columns and just have the planner remove these quals (when
they're not needed) for all queries, not just when optimizing Min/Max
aggregates.

Additionally, here we also detect IS NULL quals on a NOT NULL column and
transform that into a gating qual so that we don't have to perform the
scan at all.  This also works for join relations when the Var is not
nullable by any outer join.

This also helps with the self-join removal work as it must replace
strict join quals with IS NOT NULL quals to ensure equivalence with the
original query.

Author: David Rowley, Richard Guo, Andy Fan
Reviewed-by: Richard Guo, David Rowley
Discussion: https://postgr.es/m/CAApHDvqg6XZDhYRPz0zgOcevSMo0d3vxA9DvHrZtKfqO30WTnw@mail.gmail.com
Discussion: https://postgr.es/m/17540-7aa1855ad5ec18b4%40postgresql.org

src/backend/optimizer/plan/initsplan.c

@@ -2618,6 +2618,193 @@ check_redundant_nullability_qual(PlannerInfo *root, Node *clause)
 	return false;
 }
 
+/*
+ * add_base_clause_to_rel
+ *		Add 'restrictinfo' as a baserestrictinfo to the base relation denoted
+ *		by 'relid'.  We offer some simple prechecks to try to determine if the
+ *		qual is always true, in which case we ignore it rather than add it.
+ *		If we detect the qual is always false, we replace it with
+ *		constant-FALSE.
+ */
+static void
+add_base_clause_to_rel(PlannerInfo *root, Index relid,
+					   RestrictInfo *restrictinfo)
+{
+	RelOptInfo *rel = find_base_rel(root, relid);
+
+	Assert(bms_membership(restrictinfo->required_relids) == BMS_SINGLETON);
+
+	/* Don't add the clause if it is always true */
+	if (restriction_is_always_true(root, restrictinfo))
+		return;
+
+	/*
+	 * Substitute the origin qual with constant-FALSE if it is provably always
+	 * false.  Note that we keep the same rinfo_serial.
+	 */
+	if (restriction_is_always_false(root, restrictinfo))
+	{
+		int			save_rinfo_serial = restrictinfo->rinfo_serial;
+
+		restrictinfo = make_restrictinfo(root,
+										 (Expr *) makeBoolConst(false, false),
+										 restrictinfo->is_pushed_down,
+										 restrictinfo->has_clone,
+										 restrictinfo->is_clone,
+										 restrictinfo->pseudoconstant,
+										 0, /* security_level */
+										 restrictinfo->required_relids,
+										 restrictinfo->incompatible_relids,
+										 restrictinfo->outer_relids);
+		restrictinfo->rinfo_serial = save_rinfo_serial;
+	}
+
+	/* Add clause to rel's restriction list */
+	rel->baserestrictinfo = lappend(rel->baserestrictinfo, restrictinfo);
+
+	/* Update security level info */
+	rel->baserestrict_min_security = Min(rel->baserestrict_min_security,
+										 restrictinfo->security_level);
+}
+
+/*
+ * expr_is_nonnullable
+ *	  Check to see if the Expr cannot be NULL
+ *
+ * If the Expr is a simple Var that is defined NOT NULL and meanwhile is not
+ * nulled by any outer joins, then we can know that it cannot be NULL.
+ */
+static bool
+expr_is_nonnullable(PlannerInfo *root, Expr *expr)
+{
+	RelOptInfo *rel;
+	Var		   *var;
+
+	/* For now only check simple Vars */
+	if (!IsA(expr, Var))
+		return false;
+
+	var = (Var *) expr;
+
+	/* could the Var be nulled by any outer joins? */
+	if (!bms_is_empty(var->varnullingrels))
+		return false;
+
+	/* system columns cannot be NULL */
+	if (var->varattno < 0)
+		return true;
+
+	/* is the column defined NOT NULL? */
+	rel = find_base_rel(root, var->varno);
+	if (var->varattno > 0 &&
+		bms_is_member(var->varattno, rel->notnullattnums))
+		return true;
+
+	return false;
+}
+
+/*
+ * restriction_is_always_true
+ *	  Check to see if the RestrictInfo is always true.
+ *
+ * Currently we only check for NullTest quals and OR clauses that include
+ * NullTest quals.  We may extend it in the future.
+ */
+bool
+restriction_is_always_true(PlannerInfo *root,
+						   RestrictInfo *restrictinfo)
+{
+	/* Check for NullTest qual */
+	if (IsA(restrictinfo->clause, NullTest))
+	{
+		NullTest   *nulltest = (NullTest *) restrictinfo->clause;
+
+		/* is this NullTest an IS_NOT_NULL qual? */
+		if (nulltest->nulltesttype != IS_NOT_NULL)
+			return false;
+
+		return expr_is_nonnullable(root, nulltest->arg);
+	}
+
+	/* If it's an OR, check its sub-clauses */
+	if (restriction_is_or_clause(restrictinfo))
+	{
+		ListCell   *lc;
+
+		Assert(is_orclause(restrictinfo->orclause));
+
+		/*
+		 * if any of the given OR branches is provably always true then the
+		 * entire condition is true.
+		 */
+		foreach(lc, ((BoolExpr *) restrictinfo->orclause)->args)
+		{
+			Node	   *orarg = (Node *) lfirst(lc);
+
+			if (!IsA(orarg, RestrictInfo))
+				continue;
+
+			if (restriction_is_always_true(root, (RestrictInfo *) orarg))
+				return true;
+		}
+	}
+
+	return false;
+}
+
+/*
+ * restriction_is_always_false
+ *	  Check to see if the RestrictInfo is always false.
+ *
+ * Currently we only check for NullTest quals and OR clauses that include
+ * NullTest quals.  We may extend it in the future.
+ */
+bool
+restriction_is_always_false(PlannerInfo *root,
+							RestrictInfo *restrictinfo)
+{
+	/* Check for NullTest qual */
+	if (IsA(restrictinfo->clause, NullTest))
+	{
+		NullTest   *nulltest = (NullTest *) restrictinfo->clause;
+
+		/* is this NullTest an IS_NULL qual? */
+		if (nulltest->nulltesttype != IS_NULL)
+			return false;
+
+		return expr_is_nonnullable(root, nulltest->arg);
+	}
+
+	/* If it's an OR, check its sub-clauses */
+	if (restriction_is_or_clause(restrictinfo))
+	{
+		ListCell   *lc;
+
+		Assert(is_orclause(restrictinfo->orclause));
+
+		/*
+		 * Currently, when processing OR expressions, we only return true when
+		 * all of the OR branches are always false.  This could perhaps be
+		 * expanded to remove OR branches that are provably false.  This may
+		 * be a useful thing to do as it could result in the OR being left
+		 * with a single arg.  That's useful as it would allow the OR
+		 * condition to be replaced with its single argument which may allow
+		 * use of an index for faster filtering on the remaining condition.
+		 */
+		foreach(lc, ((BoolExpr *) restrictinfo->orclause)->args)
+		{
+			Node	   *orarg = (Node *) lfirst(lc);
+
+			if (!IsA(orarg, RestrictInfo) ||
+				!restriction_is_always_false(root, (RestrictInfo *) orarg))
+				return false;
+		}
+		return true;
+	}
+
+	return false;
+}
+
 /*
  * distribute_restrictinfo_to_rels
  *	  Push a completed RestrictInfo into the proper restriction or join
@@ -2632,7 +2819,6 @@ distribute_restrictinfo_to_rels(PlannerInfo *root,
 								RestrictInfo *restrictinfo)
 {
 	Relids		relids = restrictinfo->required_relids;
-	RelOptInfo *rel;
 
 	if (!bms_is_empty(relids))
 	{
@@ -2644,14 +2830,7 @@ distribute_restrictinfo_to_rels(PlannerInfo *root,
 			 * There is only one relation participating in the clause, so it
 			 * is a restriction clause for that relation.
 			 */
-			rel = find_base_rel(root, relid);
-
-			/* Add clause to rel's restriction list */
-			rel->baserestrictinfo = lappend(rel->baserestrictinfo,
-											restrictinfo);
-			/* Update security level info */
-			rel->baserestrict_min_security = Min(rel->baserestrict_min_security,
-												 restrictinfo->security_level);
+			add_base_clause_to_rel(root, relid, restrictinfo);
 		}
 		else
 		{