Fix some planner issues found while investigating Kevin Grittner's report

of poorer planning in 8.3 than 8.2:

1. After pushing a constant across an outer join --- ie, given
"a LEFT JOIN b ON (a.x = b.y) WHERE a.x = 42", we can deduce that b.y is
sort of equal to 42, in the sense that we needn't fetch any b rows where
it isn't 42 --- loop to see if any additional deductions can be made.
Previous releases did that by recursing, but I had mistakenly thought that
this was no longer necessary given the EquivalenceClass machinery.

2. Allow pushing constants across outer join conditions even if the
condition is outerjoin_delayed due to a lower outer join.  This is safe
as long as the condition is strict and we re-test it at the upper join.

3. Keep the outer-join clause even if we successfully push a constant
across it.  This is *necessary* in the outerjoin_delayed case, but
even in the simple case, it seems better to do this to ensure that the
join search order heuristics will consider the join as reasonable to
make.  Mark such a clause as having selectivity 1.0, though, since it's
not going to eliminate very many rows after application of the constant
condition.

4. Tweak have_relevant_eclass_joinclause to report that two relations
are joinable when they have vars that are equated to the same constant.
We won't actually generate any joinclause from such an EquivalenceClass,
but again it seems that in such a case it's a good idea to consider
the join as worth costing out.

5. Fix a bug in select_mergejoin_clauses that was exposed by these
changes: we have to reject candidate mergejoin clauses if either side was
equated to a constant, because we can't construct a canonical pathkey list
for such a clause.  This is an implementation restriction that might be
worth fixing someday, but it doesn't seem critical to get it done for 8.3.

Author: tglsfdc

src/backend/optimizer/path/equivclass.c

@@ -10,7 +10,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.8 2008/01/01 19:45:50 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.9 2008/01/09 20:42:27 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -52,10 +52,10 @@ static RestrictInfo *create_join_clause(PlannerInfo *root,
 				   EquivalenceMember *leftem,
 				   EquivalenceMember *rightem,
 				   EquivalenceClass *parent_ec);
-static void reconsider_outer_join_clause(PlannerInfo *root,
+static bool reconsider_outer_join_clause(PlannerInfo *root,
 							 RestrictInfo *rinfo,
 							 bool outer_on_left);
-static void reconsider_full_join_clause(PlannerInfo *root,
+static bool reconsider_full_join_clause(PlannerInfo *root,
 							RestrictInfo *rinfo);
 
 
@@ -1094,8 +1094,9 @@ create_join_clause(PlannerInfo *root,
 /*
  * reconsider_outer_join_clauses
  *	  Re-examine any outer-join clauses that were set aside by
- *	  distribute_qual_to_rels(), and either create EquivalenceClasses
- *	  to replace them or push them out into the regular join-clause lists.
+ *	  distribute_qual_to_rels(), and see if we can derive any
+ *	  EquivalenceClasses from them.  Then, if they were not made
+ *	  redundant, push them out into the regular join-clause lists.
  *
  * When we have mergejoinable clauses A = B that are outer-join clauses,
  * we can't blindly combine them with other clauses A = C to deduce B = C,
@@ -1112,8 +1113,8 @@ create_join_clause(PlannerInfo *root,
  * equivalence clause.)
  *
  * Note that the above rule does not work for full outer joins; nor is it
- * very interesting to consider cases where the equivalence clause involves
- * relations entirely outside the outer join, since such clauses couldn't
+ * very interesting to consider cases where the generated equivalence clause
+ * would involve relations outside the outer join, since such clauses couldn't
  * be pushed into the inner side's scan anyway.  So the restriction to
  * outervar = pseudoconstant is not really giving up anything.
  *
@@ -1141,57 +1142,161 @@ create_join_clause(PlannerInfo *root,
  * that could result in propagating constant restrictions from
  * INNERVAR to OUTERVAR, which would be very wrong.
  *
+ * It's possible that the INNERVAR is also an OUTERVAR for some other
+ * outer-join clause, in which case the process can be repeated.  So we repeat
+ * looping over the lists of clauses until no further deductions can be made.
+ * Whenever we do make a deduction, we remove the generating clause from the
+ * lists, since we don't want to make the same deduction twice.
+ *
  * If we don't find any match for a set-aside outer join clause, we must
  * throw it back into the regular joinclause processing by passing it to
- * distribute_restrictinfo_to_rels().
+ * distribute_restrictinfo_to_rels().  If we do generate a derived clause,
+ * however, the outer-join clause is redundant.  We still throw it back,
+ * because otherwise the join will be seen as a clauseless join and avoided
+ * during join order searching; but we mark it as redundant to keep from
+ * messing up the joinrel's size estimate.  (This behavior means that the
+ * API for this routine is uselessly complex: we could have just put all
+ * the clauses into the regular processing initially.  We keep it because
+ * someday we might want to do something else, such as inserting "dummy"
+ * joinclauses instead of real ones.)
+ *
+ * Outer join clauses that are marked outerjoin_delayed are special: this
+ * condition means that one or both VARs might go to null due to a lower
+ * outer join.  We can still push a constant through the clause, but only
+ * if its operator is strict; and we *have to* throw the clause back into
+ * regular joinclause processing.  By keeping the strict join clause,
+ * we ensure that any null-extended rows that are mistakenly generated due
+ * to suppressing rows not matching the constant will be rejected at the
+ * upper outer join.  (This doesn't work for full-join clauses.)
  */
 void
 reconsider_outer_join_clauses(PlannerInfo *root)
 {
-	ListCell   *lc;
+	bool		found;
+	ListCell   *cell;
+	ListCell   *prev;
+	ListCell   *next;
+
+	/* Outer loop repeats until we find no more deductions */
+	do
+	{
+		found = false;
+
+		/* Process the LEFT JOIN clauses */
+		prev = NULL;
+		for (cell = list_head(root->left_join_clauses); cell; cell = next)
+		{
+			RestrictInfo *rinfo = (RestrictInfo *) lfirst(cell);
+
+			next = lnext(cell);
+			if (reconsider_outer_join_clause(root, rinfo, true))
+			{
+				found = true;
+				/* remove it from the list */
+				root->left_join_clauses =
+					list_delete_cell(root->left_join_clauses, cell, prev);
+				/* we throw it back anyway (see notes above) */
+				/* but the thrown-back clause has no extra selectivity */
+				rinfo->this_selec = 1.0;
+				distribute_restrictinfo_to_rels(root, rinfo);
+			}
+			else
+				prev = cell;
+		}
+
+		/* Process the RIGHT JOIN clauses */
+		prev = NULL;
+		for (cell = list_head(root->right_join_clauses); cell; cell = next)
+		{
+			RestrictInfo *rinfo = (RestrictInfo *) lfirst(cell);
+
+			next = lnext(cell);
+			if (reconsider_outer_join_clause(root, rinfo, false))
+			{
+				found = true;
+				/* remove it from the list */
+				root->right_join_clauses =
+					list_delete_cell(root->right_join_clauses, cell, prev);
+				/* we throw it back anyway (see notes above) */
+				/* but the thrown-back clause has no extra selectivity */
+				rinfo->this_selec = 1.0;
+				distribute_restrictinfo_to_rels(root, rinfo);
+			}
+			else
+				prev = cell;
+		}
+
+		/* Process the FULL JOIN clauses */
+		prev = NULL;
+		for (cell = list_head(root->full_join_clauses); cell; cell = next)
+		{
+			RestrictInfo *rinfo = (RestrictInfo *) lfirst(cell);
+
+			next = lnext(cell);
+			if (reconsider_full_join_clause(root, rinfo))
+			{
+				found = true;
+				/* remove it from the list */
+				root->full_join_clauses =
+					list_delete_cell(root->full_join_clauses, cell, prev);
+				/* we throw it back anyway (see notes above) */
+				/* but the thrown-back clause has no extra selectivity */
+				rinfo->this_selec = 1.0;
+				distribute_restrictinfo_to_rels(root, rinfo);
+			}
+			else
+				prev = cell;
+		}
+	} while (found);
 
-	/* Process the LEFT JOIN clauses */
-	foreach(lc, root->left_join_clauses)
+	/* Now, any remaining clauses have to be thrown back */
+	foreach(cell, root->left_join_clauses)
 	{
-		RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
+		RestrictInfo *rinfo = (RestrictInfo *) lfirst(cell);
 
-		reconsider_outer_join_clause(root, rinfo, true);
+		distribute_restrictinfo_to_rels(root, rinfo);
 	}
-	/* And the RIGHT JOIN clauses */
-	foreach(lc, root->right_join_clauses)
+	foreach(cell, root->right_join_clauses)
 	{
-		RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
+		RestrictInfo *rinfo = (RestrictInfo *) lfirst(cell);
 
-		reconsider_outer_join_clause(root, rinfo, false);
+		distribute_restrictinfo_to_rels(root, rinfo);
 	}
-	/* And the FULL JOIN clauses */
-	foreach(lc, root->full_join_clauses)
+	foreach(cell, root->full_join_clauses)
 	{
-		RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
+		RestrictInfo *rinfo = (RestrictInfo *) lfirst(cell);
 
-		reconsider_full_join_clause(root, rinfo);
+		distribute_restrictinfo_to_rels(root, rinfo);
 	}
 }
 
 /*
  * reconsider_outer_join_clauses for a single LEFT/RIGHT JOIN clause
+ *
+ * Returns TRUE if we were able to propagate a constant through the clause.
  */
-static void
+static bool
 reconsider_outer_join_clause(PlannerInfo *root, RestrictInfo *rinfo,
 							 bool outer_on_left)
 {
 	Expr	   *outervar,
 			   *innervar;
-	Oid			left_type,
+	Oid			opno,
+				left_type,
 				right_type,
 				inner_datatype;
 	Relids		inner_relids;
 	ListCell   *lc1;
 
-	/* Extract needed info from the clause */
 	Assert(is_opclause(rinfo->clause));
-	op_input_types(((OpExpr *) rinfo->clause)->opno,
-				   &left_type, &right_type);
+	opno = ((OpExpr *) rinfo->clause)->opno;
+
+	/* If clause is outerjoin_delayed, operator must be strict */
+	if (rinfo->outerjoin_delayed && !op_strict(opno))
+		return false;
+
+	/* Extract needed info from the clause */
+	op_input_types(opno, &left_type, &right_type);
 	if (outer_on_left)
 	{
 		outervar = (Expr *) get_leftop(rinfo->clause);
@@ -1266,41 +1371,46 @@ reconsider_outer_join_clause(PlannerInfo *root, RestrictInfo *rinfo,
 		}
 
 		/*
-		 * If we were able to equate INNERVAR to any constant, we're done, and
-		 * we can throw away the outer-join clause as redundant.  Otherwise,
-		 * fall out of the search loop, since we know the OUTERVAR appears in
-		 * at most one EC.
+		 * If we were able to equate INNERVAR to any constant, report success.
+		 * Otherwise, fall out of the search loop, since we know the OUTERVAR
+		 * appears in at most one EC.
 		 */
 		if (match)
-			return;
+			return true;
 		else
 			break;
 	}
 
-	/* We did not find a match, so throw it back into regular processing */
-	distribute_restrictinfo_to_rels(root, rinfo);
+	return false;				/* failed to make any deduction */
 }
 
 /*
  * reconsider_outer_join_clauses for a single FULL JOIN clause
+ *
+ * Returns TRUE if we were able to propagate a constant through the clause.
  */
-static void
+static bool
 reconsider_full_join_clause(PlannerInfo *root, RestrictInfo *rinfo)
 {
 	Expr	   *leftvar;
 	Expr	   *rightvar;
-	Oid			left_type,
+	Oid			opno,
+				left_type,
 				right_type;
 	Relids		left_relids,
 				right_relids;
 	ListCell   *lc1;
 
+	/* Can't use an outerjoin_delayed clause here */
+	if (rinfo->outerjoin_delayed)
+		return false;
+
 	/* Extract needed info from the clause */
 	Assert(is_opclause(rinfo->clause));
+	opno = ((OpExpr *) rinfo->clause)->opno;
+	op_input_types(opno, &left_type, &right_type);
 	leftvar = (Expr *) get_leftop(rinfo->clause);
 	rightvar = (Expr *) get_rightop(rinfo->clause);
-	op_input_types(((OpExpr *) rinfo->clause)->opno,
-				   &left_type, &right_type);
 	left_relids = rinfo->left_relids;
 	right_relids = rinfo->right_relids;
 
@@ -1412,7 +1522,7 @@ reconsider_full_join_clause(PlannerInfo *root, RestrictInfo *rinfo)
 		if (matchleft && matchright)
 		{
 			cur_ec->ec_members = list_delete_ptr(cur_ec->ec_members, coal_em);
-			return;
+			return true;
 		}
 
 		/*
@@ -1423,8 +1533,7 @@ reconsider_full_join_clause(PlannerInfo *root, RestrictInfo *rinfo)
 		break;
 	}
 
-	/* We did not find a match, so throw it back into regular processing */
-	distribute_restrictinfo_to_rels(root, rinfo);
+	return false;				/* failed to make any deduction */
 }
 
 
@@ -1651,16 +1760,22 @@ have_relevant_eclass_joinclause(PlannerInfo *root,
 		ListCell   *lc2;
 
 		/*
-		 * Won't generate joinclauses if const or single-member (the latter
-		 * test covers the volatile case too)
+		 * Won't generate joinclauses if single-member (this test covers the
+		 * volatile case too)
 		 */
-		if (ec->ec_has_const || list_length(ec->ec_members) <= 1)
+		if (list_length(ec->ec_members) <= 1)
 			continue;
 
 		/*
 		 * Note we don't test ec_broken; if we did, we'd need a separate code
 		 * path to look through ec_sources.  Checking the members anyway is OK
 		 * as a possibly-overoptimistic heuristic.
+		 *
+		 * We don't test ec_has_const either, even though a const eclass
+		 * won't generate real join clauses.  This is because if we had
+		 * "WHERE a.x = b.y and a.x = 42", it is worth considering a join
+		 * between a and b, since the join result is likely to be small even
+		 * though it'll end up being an unqualified nestloop.
 		 */
 
 		/* Needn't scan if it couldn't contain members from each rel */
@@ -1674,8 +1789,8 @@ have_relevant_eclass_joinclause(PlannerInfo *root,
 		{
 			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2);
 
-			if (cur_em->em_is_child)
-				continue;		/* ignore children here */
+			if (cur_em->em_is_const || cur_em->em_is_child)
+				continue;		/* ignore consts and children here */
 			if (bms_is_subset(cur_em->em_relids, rel1->relids))
 			{
 				has_rel1 = true;
@@ -1719,16 +1834,22 @@ has_relevant_eclass_joinclause(PlannerInfo *root, RelOptInfo *rel1)
 		ListCell   *lc2;
 
 		/*
-		 * Won't generate joinclauses if const or single-member (the latter
-		 * test covers the volatile case too)
+		 * Won't generate joinclauses if single-member (this test covers the
+		 * volatile case too)
 		 */
-		if (ec->ec_has_const || list_length(ec->ec_members) <= 1)
+		if (list_length(ec->ec_members) <= 1)
 			continue;
 
 		/*
 		 * Note we don't test ec_broken; if we did, we'd need a separate code
 		 * path to look through ec_sources.  Checking the members anyway is OK
 		 * as a possibly-overoptimistic heuristic.
+		 *
+		 * We don't test ec_has_const either, even though a const eclass
+		 * won't generate real join clauses.  This is because if we had
+		 * "WHERE a.x = b.y and a.x = 42", it is worth considering a join
+		 * between a and b, since the join result is likely to be small even
+		 * though it'll end up being an unqualified nestloop.
 		 */
 
 		/* Needn't scan if it couldn't contain members from each rel */
@@ -1742,8 +1863,8 @@ has_relevant_eclass_joinclause(PlannerInfo *root, RelOptInfo *rel1)
 		{
 			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2);
 
-			if (cur_em->em_is_child)
-				continue;		/* ignore children here */
+			if (cur_em->em_is_const || cur_em->em_is_child)
+				continue;		/* ignore consts and children here */
 			if (bms_is_subset(cur_em->em_relids, rel1->relids))
 			{
 				has_rel1 = true;