Revise the planner's handling of "pseudoconstant" WHERE clauses, that is

clauses containing no variables and no volatile functions.  Such a clause
can be used as a one-time qual in a gating Result plan node, to suppress
plan execution entirely when it is false.  Even when the clause is true,
putting it in a gating node wins by avoiding repeated evaluation of the
clause.  In previous PG releases, query_planner() would do this for
pseudoconstant clauses appearing at the top level of the jointree, but
there was no ability to generate a gating Result deeper in the plan tree.
To fix it, get rid of the special case in query_planner(), and instead
process pseudoconstant clauses through the normal RestrictInfo qual
distribution mechanism.  When a pseudoconstant clause is found attached to
a path node in create_plan(), pull it out and generate a gating Result at
that point.  This requires special-casing pseudoconstants in selectivity
estimation and cost_qual_eval, but on the whole it's pretty clean.
It probably even makes the planner a bit faster than before for the normal
case of no pseudoconstants, since removing pull_constant_clauses saves one
useless traversal of the qual tree.  Per gripe from Phil Frost.

Author: tglsfdc

src/backend/nodes/copyfuncs.c

@@ -15,7 +15,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/nodes/copyfuncs.c,v 1.337 2006/06/27 03:43:19 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/nodes/copyfuncs.c,v 1.338 2006/07/01 18:38:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1264,6 +1264,7 @@ _copyRestrictInfo(RestrictInfo *from)
 	COPY_SCALAR_FIELD(is_pushed_down);
 	COPY_SCALAR_FIELD(outerjoin_delayed);
 	COPY_SCALAR_FIELD(can_join);
+	COPY_SCALAR_FIELD(pseudoconstant);
 	COPY_BITMAPSET_FIELD(clause_relids);
 	COPY_BITMAPSET_FIELD(required_relids);
 	COPY_BITMAPSET_FIELD(left_relids);

src/backend/nodes/outfuncs.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/nodes/outfuncs.c,v 1.274 2006/04/30 18:30:39 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/nodes/outfuncs.c,v 1.275 2006/07/01 18:38:32 tgl Exp $
  *
  * NOTES
  *	  Every node type that can appear in stored rules' parsetrees *must*
@@ -1107,8 +1107,7 @@ _outResultPath(StringInfo str, ResultPath *node)
 
 	_outPathInfo(str, (Path *) node);
 
-	WRITE_NODE_FIELD(subpath);
-	WRITE_NODE_FIELD(constantqual);
+	WRITE_NODE_FIELD(quals);
 }
 
 static void
@@ -1185,6 +1184,7 @@ _outPlannerInfo(StringInfo str, PlannerInfo *node)
 	WRITE_BOOL_FIELD(hasJoinRTEs);
 	WRITE_BOOL_FIELD(hasOuterJoins);
 	WRITE_BOOL_FIELD(hasHavingQual);
+	WRITE_BOOL_FIELD(hasPseudoConstantQuals);
 }
 
 static void
@@ -1252,6 +1252,7 @@ _outRestrictInfo(StringInfo str, RestrictInfo *node)
 	WRITE_BOOL_FIELD(is_pushed_down);
 	WRITE_BOOL_FIELD(outerjoin_delayed);
 	WRITE_BOOL_FIELD(can_join);
+	WRITE_BOOL_FIELD(pseudoconstant);
 	WRITE_BITMAPSET_FIELD(clause_relids);
 	WRITE_BITMAPSET_FIELD(required_relids);
 	WRITE_BITMAPSET_FIELD(left_relids);

src/backend/optimizer/README

@@ -329,7 +329,7 @@ RelOptInfo      - a relation or joined relations
   BitmapHeapPath - top of a bitmapped index scan
   TidPath       - scan by CTID
   AppendPath    - append multiple subpaths together
-  ResultPath    - a Result plan node (used for variable-free tlist or qual)
+  ResultPath    - a Result plan node (used for FROM-less SELECT)
   MaterialPath  - a Material plan node
   UniquePath    - remove duplicate rows
   NestPath      - nested-loop joins

src/backend/optimizer/path/allpaths.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.146 2006/05/02 04:34:18 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.147 2006/07/01 18:38:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -446,7 +446,9 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	 * There are several cases where we cannot push down clauses. Restrictions
 	 * involving the subquery are checked by subquery_is_pushdown_safe().
 	 * Restrictions on individual clauses are checked by
-	 * qual_is_pushdown_safe().
+	 * qual_is_pushdown_safe().  Also, we don't want to push down
+	 * pseudoconstant clauses; better to have the gating node above the
+	 * subquery.
 	 *
 	 * Non-pushed-down clauses will get evaluated as qpquals of the
 	 * SubqueryScan node.
@@ -466,7 +468,8 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
 			RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);
 			Node	   *clause = (Node *) rinfo->clause;
 
-			if (qual_is_pushdown_safe(subquery, rti, clause, differentTypes))
+			if (!rinfo->pseudoconstant &&
+				qual_is_pushdown_safe(subquery, rti, clause, differentTypes))
 			{
 				/* Push it down */
 				subquery_push_qual(subquery, rte, rti, clause);
@@ -1066,7 +1069,6 @@ print_path(PlannerInfo *root, Path *path, int indent)
 			break;
 		case T_ResultPath:
 			ptype = "Result";
-			subpath = ((ResultPath *) path)->subpath;
 			break;
 		case T_MaterialPath:
 			ptype = "Material";

src/backend/optimizer/path/clausesel.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.79 2006/03/07 01:00:15 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.80 2006/07/01 18:38:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -117,10 +117,18 @@ clauselist_selectivity(PlannerInfo *root,
 
 		/*
 		 * Check for being passed a RestrictInfo.
+		 *
+		 * If it's a pseudoconstant RestrictInfo, then s2 is either 1.0 or
+		 * 0.0; just use that rather than looking for range pairs.
 		 */
 		if (IsA(clause, RestrictInfo))
 		{
 			rinfo = (RestrictInfo *) clause;
+			if (rinfo->pseudoconstant)
+			{
+				s1 = s1 * s2;
+				continue;
+			}
 			clause = (Node *) rinfo->clause;
 		}
 		else
@@ -422,6 +430,20 @@ clause_selectivity(PlannerInfo *root,
 	{
 		rinfo = (RestrictInfo *) clause;
 
+		/*
+		 * If the clause is marked pseudoconstant, then it will be used as
+		 * a gating qual and should not affect selectivity estimates; hence
+		 * return 1.0.  The only exception is that a constant FALSE may
+		 * be taken as having selectivity 0.0, since it will surely mean
+		 * no rows out of the plan.  This case is simple enough that we
+		 * need not bother caching the result.
+		 */
+		if (rinfo->pseudoconstant)
+		{
+			if (! IsA(rinfo->clause, Const))
+				return s1;
+		}
+
 		/*
 		 * If possible, cache the result of the selectivity calculation for
 		 * the clause.	We can cache if varRelid is zero or the clause
@@ -509,7 +531,10 @@ clause_selectivity(PlannerInfo *root,
 	else if (IsA(clause, Const))
 	{
 		/* bool constant is pretty easy... */
-		s1 = ((bool) ((Const *) clause)->constvalue) ? 1.0 : 0.0;
+		Const  *con = (Const *) clause;
+
+		s1 = con->constisnull ? 0.0 :
+			DatumGetBool(con->constvalue) ? 1.0 : 0.0;
 	}
 	else if (IsA(clause, Param))
 	{
@@ -519,7 +544,10 @@ clause_selectivity(PlannerInfo *root,
 		if (IsA(subst, Const))
 		{
 			/* bool constant is pretty easy... */
-			s1 = ((bool) ((Const *) subst)->constvalue) ? 1.0 : 0.0;
+			Const  *con = (Const *) subst;
+
+			s1 = con->constisnull ? 0.0 :
+				DatumGetBool(con->constvalue) ? 1.0 : 0.0;
 		}
 		else
 		{

src/backend/optimizer/path/costsize.c

@@ -54,7 +54,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.158 2006/06/06 17:59:57 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.159 2006/07/01 18:38:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1604,20 +1604,29 @@ cost_qual_eval(QualCost *cost, List *quals)
 		 * routine's use, so that it's not necessary to evaluate the qual
 		 * clause's cost more than once.  If the clause's cost hasn't been
 		 * computed yet, the field's startup value will contain -1.
+		 *
+		 * If the RestrictInfo is marked pseudoconstant, it will be tested
+		 * only once, so treat its cost as all startup cost.
 		 */
 		if (qual && IsA(qual, RestrictInfo))
 		{
-			RestrictInfo *restrictinfo = (RestrictInfo *) qual;
+			RestrictInfo *rinfo = (RestrictInfo *) qual;
 
-			if (restrictinfo->eval_cost.startup < 0)
+			if (rinfo->eval_cost.startup < 0)
 			{
-				restrictinfo->eval_cost.startup = 0;
-				restrictinfo->eval_cost.per_tuple = 0;
-				cost_qual_eval_walker((Node *) restrictinfo->clause,
-									  &restrictinfo->eval_cost);
+				rinfo->eval_cost.startup = 0;
+				rinfo->eval_cost.per_tuple = 0;
+				cost_qual_eval_walker((Node *) rinfo->clause,
+									  &rinfo->eval_cost);
+				if (rinfo->pseudoconstant)
+				{
+					/* count one execution during startup */
+					rinfo->eval_cost.startup += rinfo->eval_cost.per_tuple;
+					rinfo->eval_cost.per_tuple = 0;
+				}
 			}
-			cost->startup += restrictinfo->eval_cost.startup;
-			cost->per_tuple += restrictinfo->eval_cost.per_tuple;
+			cost->startup += rinfo->eval_cost.startup;
+			cost->per_tuple += rinfo->eval_cost.per_tuple;
 		}
 		else
 		{
@@ -1876,7 +1885,9 @@ set_joinrel_size_estimates(PlannerInfo *root, RelOptInfo *rel,
 	 *
 	 * If we are doing an outer join, take that into account: the output must
 	 * be at least as large as the non-nullable input.	(Is there any chance
-	 * of being even smarter?)
+	 * of being even smarter?)  (XXX this is not really right, because it
+	 * assumes all the restriction clauses are join clauses; we should figure
+	 * pushed-down clauses separately.)
 	 *
 	 * For JOIN_IN and variants, the Cartesian product is figured with respect
 	 * to a unique-ified input, and then we can clamp to the size of the other

src/backend/optimizer/path/indxpath.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.208 2006/06/07 17:08:07 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.209 2006/07/01 18:38:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -998,6 +998,15 @@ match_clause_to_indexcol(IndexOptInfo *index,
 	Oid			expr_op;
 	bool		plain_op;
 
+	/*
+	 * Never match pseudoconstants to indexes.  (Normally this could not
+	 * happen anyway, since a pseudoconstant clause couldn't contain a
+	 * Var, but what if someone builds an expression index on a constant?
+	 * It's not totally unreasonable to do so with a partial index, either.)
+	 */
+	if (rinfo->pseudoconstant)
+		return false;
+
 	/* First check for boolean-index cases. */
 	if (IsBooleanOpclass(opclass))
 	{
@@ -2212,6 +2221,7 @@ expand_indexqual_conditions(IndexOptInfo *index, List *clausegroups)
 										  make_restrictinfo(boolqual,
 															true,
 															false,
+															false,
 															NULL));
 					continue;
 				}
@@ -2577,7 +2587,7 @@ expand_indexqual_rowcompare(RestrictInfo *rinfo,
 								  matching_cols);
 		rc->rargs = list_truncate((List *) copyObject(clause->rargs),
 								  matching_cols);
-		return make_restrictinfo((Expr *) rc, true, false, NULL);
+		return make_restrictinfo((Expr *) rc, true, false, false, NULL);
 	}
 	else
 	{
@@ -2586,7 +2596,7 @@ expand_indexqual_rowcompare(RestrictInfo *rinfo,
 		opexpr = make_opclause(linitial_oid(new_ops), BOOLOID, false,
 							   copyObject(linitial(clause->largs)),
 							   copyObject(linitial(clause->rargs)));
-		return make_restrictinfo(opexpr, true, false, NULL);
+		return make_restrictinfo(opexpr, true, false, false, NULL);
 	}
 }
 
@@ -2678,7 +2688,7 @@ prefix_quals(Node *leftop, Oid opclass,
 			elog(ERROR, "no = operator for opclass %u", opclass);
 		expr = make_opclause(oproid, BOOLOID, false,
 							 (Expr *) leftop, (Expr *) prefix_const);
-		result = list_make1(make_restrictinfo(expr, true, false, NULL));
+		result = list_make1(make_restrictinfo(expr, true, false, false, NULL));
 		return result;
 	}
 
@@ -2693,7 +2703,7 @@ prefix_quals(Node *leftop, Oid opclass,
 		elog(ERROR, "no >= operator for opclass %u", opclass);
 	expr = make_opclause(oproid, BOOLOID, false,
 						 (Expr *) leftop, (Expr *) prefix_const);
-	result = list_make1(make_restrictinfo(expr, true, false, NULL));
+	result = list_make1(make_restrictinfo(expr, true, false, false, NULL));
 
 	/*-------
 	 * If we can create a string larger than the prefix, we can say
@@ -2709,7 +2719,8 @@ prefix_quals(Node *leftop, Oid opclass,
 			elog(ERROR, "no < operator for opclass %u", opclass);
 		expr = make_opclause(oproid, BOOLOID, false,
 							 (Expr *) leftop, (Expr *) greaterstr);
-		result = lappend(result, make_restrictinfo(expr, true, false, NULL));
+		result = lappend(result,
+						 make_restrictinfo(expr, true, false, false, NULL));
 	}
 
 	return result;
@@ -2772,7 +2783,7 @@ network_prefix_quals(Node *leftop, Oid expr_op, Oid opclass, Datum rightop)
 						 (Expr *) leftop,
 						 (Expr *) makeConst(datatype, -1, opr1right,
 											false, false));
-	result = list_make1(make_restrictinfo(expr, true, false, NULL));
+	result = list_make1(make_restrictinfo(expr, true, false, false, NULL));
 
 	/* create clause "key <= network_scan_last( rightop )" */
 
@@ -2787,7 +2798,8 @@ network_prefix_quals(Node *leftop, Oid expr_op, Oid opclass, Datum rightop)
 						 (Expr *) leftop,
 						 (Expr *) makeConst(datatype, -1, opr2right,
 											false, false));
-	result = lappend(result, make_restrictinfo(expr, true, false, NULL));
+	result = lappend(result,
+					 make_restrictinfo(expr, true, false, false, NULL));
 
 	return result;
 }