Speedup child EquivalenceMember lookup in planner

When planning queries to partitioned tables, we clone all
EquivalenceMembers belonging to the partitioned table into em_is_child
EquivalenceMembers for each non-pruned partition.  For partitioned tables
with large numbers of partitions, this meant the ec_members list could
become large and code searching that list would become slow.  Effectively,
the more partitions which were present, the more searches needed to be
performed for operations such as find_ec_member_matching_expr() during
create_plan() and the more partitions present, the longer these searches
would take, i.e., a quadratic slowdown.

To fix this, here we adjust how we store EquivalenceMembers for
em_is_child members.  Instead of storing these directly in ec_members,
these are now stored in a new array of Lists in the EquivalenceClass,
which is indexed by the relid.  When we want to find EquivalenceMembers
belonging to a certain child relation, we can narrow the search to the
array element for that relation.

To make EquivalenceMember lookup easier and to reduce the amount of code
change, this commit provides a pair of functions to allow iteration over
the EquivalenceMembers of an EC which also handles finding the child
members, if required.  Callers that never need to look at child members
can remain using the foreach loop over ec_members, which will now often
be faster due to only parent-level members being stored there.

The actual performance increases here are highly dependent on the number
of partitions and the query being planned.  Performance increases can be
visible with as few as 8 partitions, but the speedup is marginal for
such low numbers of partitions.  The speedups become much more visible
with a few dozen to hundreds of partitions.  With some tested queries
using 56 partitions, the planner was around 3x faster than before.  For
use cases with thousands of partitions, these are likely to become
significantly faster.  Some testing has shown planner speedups of 60x or
more with 8192 partitions.

Author: Yuya Watari <watari.yuya@gmail.com>
Co-authored-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
Reviewed-by: Alena Rybakina <lena.ribackina@yandex.ru>
Reviewed-by: Dmitry Dolgov <9erthalion6@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Tested-by: Thom Brown <thom@linux.com>
Tested-by: newtglobal postgresql_contributors <postgresql_contributors@newtglobalcorp.com>
Discussion: https://postgr.es/m/CAJ2pMkZNCgoUKSE%2B_5LthD%2BKbXKvq6h2hQN8Esxpxd%2Bcxmgomg%40mail.gmail.com

contrib/postgres_fdw/postgres_fdw.c

@@ -7847,14 +7847,13 @@ conversion_error_callback(void *arg)
 EquivalenceMember *
 find_em_for_rel(PlannerInfo *root, EquivalenceClass *ec, RelOptInfo *rel)
 {
-	ListCell   *lc;
-
 	PgFdwRelationInfo *fpinfo = (PgFdwRelationInfo *) rel->fdw_private;
+	EquivalenceMemberIterator it;
+	EquivalenceMember *em;
 
-	foreach(lc, ec->ec_members)
+	setup_eclass_member_iterator(&it, ec, rel->relids);
+	while ((em = eclass_member_iterator_next(&it)) != NULL)
 	{
-		EquivalenceMember *em = (EquivalenceMember *) lfirst(lc);
-
 		/*
 		 * Note we require !bms_is_empty, else we'd accept constant
 		 * expressions which are not suitable for the purpose.
@@ -7908,7 +7907,10 @@ find_em_for_rel_target(PlannerInfo *root, EquivalenceClass *ec,
 		while (expr && IsA(expr, RelabelType))
 			expr = ((RelabelType *) expr)->arg;
 
-		/* Locate an EquivalenceClass member matching this expr, if any */
+		/*
+		 * Locate an EquivalenceClass member matching this expr, if any.
+		 * Ignore child members.
+		 */
 		foreach(lc2, ec->ec_members)
 		{
 			EquivalenceMember *em = (EquivalenceMember *) lfirst(lc2);
@@ -7918,9 +7920,8 @@ find_em_for_rel_target(PlannerInfo *root, EquivalenceClass *ec,
 			if (em->em_is_const)
 				continue;
 
-			/* Ignore child members */
+			/* Child members should not exist in ec_members */
-			if (em->em_is_child)
+			Assert(!em->em_is_child);
-				continue;
 
 			/* Match if same expression (after stripping relabel) */
 			em_expr = em->em_expr;

src/backend/nodes/outfuncs.c

@@ -465,7 +465,9 @@ _outEquivalenceClass(StringInfo str, const EquivalenceClass *node)
 
 	WRITE_NODE_FIELD(ec_opfamilies);
 	WRITE_OID_FIELD(ec_collation);
+	WRITE_INT_FIELD(ec_childmembers_size);
 	WRITE_NODE_FIELD(ec_members);
+	WRITE_NODE_ARRAY(ec_childmembers, node->ec_childmembers_size);
 	WRITE_NODE_FIELD(ec_sources);
 	/* Only ec_derives_list is written; hash is not serialized. */
 	WRITE_NODE_FIELD(ec_derives_list);

src/backend/optimizer/path/equivclass.c

@@ -34,11 +34,23 @@
 #include "utils/lsyscache.h"
 
 
+static EquivalenceMember *make_eq_member(EquivalenceClass *ec,
+										 Expr *expr, Relids relids,
+										 JoinDomain *jdomain,
+										 EquivalenceMember *parent,
+										 Oid datatype);
 static EquivalenceMember *add_eq_member(EquivalenceClass *ec,
 										Expr *expr, Relids relids,
 										JoinDomain *jdomain,
-										EquivalenceMember *parent,
 										Oid datatype);
+static EquivalenceMember *add_child_eq_member(PlannerInfo *root,
+											  EquivalenceClass *ec,
+											  int ec_index, Expr *expr,
+											  Relids relids,
+											  JoinDomain *jdomain,
+											  EquivalenceMember *parent_em,
+											  Oid datatype,
+											  Index child_relid);
 static void generate_base_implied_equalities_const(PlannerInfo *root,
 												   EquivalenceClass *ec);
 static void generate_base_implied_equalities_no_const(PlannerInfo *root,
@@ -314,11 +326,15 @@ process_equivalence(PlannerInfo *root,
 		if (!equal(opfamilies, cur_ec->ec_opfamilies))
 			continue;
 
+		/* We don't expect any children yet */
+		Assert(cur_ec->ec_childmembers == NULL);
+
 		foreach(lc2, cur_ec->ec_members)
 		{
 			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2);
 
-			Assert(!cur_em->em_is_child);	/* no children yet */
+			/* Child members should not exist in ec_members */
+			Assert(!cur_em->em_is_child);
 
 			/*
 			 * Match constants only within the same JoinDomain (see
@@ -428,7 +444,7 @@ process_equivalence(PlannerInfo *root,
 	{
 		/* Case 3: add item2 to ec1 */
 		em2 = add_eq_member(ec1, item2, item2_relids,
-							jdomain, NULL, item2_type);
+							jdomain, item2_type);
 		ec1->ec_sources = lappend(ec1->ec_sources, restrictinfo);
 		ec1->ec_min_security = Min(ec1->ec_min_security,
 								   restrictinfo->security_level);
@@ -445,7 +461,7 @@ process_equivalence(PlannerInfo *root,
 	{
 		/* Case 3: add item1 to ec2 */
 		em1 = add_eq_member(ec2, item1, item1_relids,
-							jdomain, NULL, item1_type);
+							jdomain, item1_type);
 		ec2->ec_sources = lappend(ec2->ec_sources, restrictinfo);
 		ec2->ec_min_security = Min(ec2->ec_min_security,
 								   restrictinfo->security_level);
@@ -465,7 +481,9 @@ process_equivalence(PlannerInfo *root,
 
 		ec->ec_opfamilies = opfamilies;
 		ec->ec_collation = collation;
+		ec->ec_childmembers_size = 0;
 		ec->ec_members = NIL;
+		ec->ec_childmembers = NULL;
 		ec->ec_sources = list_make1(restrictinfo);
 		ec->ec_derives_list = NIL;
 		ec->ec_derives_hash = NULL;
@@ -478,9 +496,9 @@ process_equivalence(PlannerInfo *root,
 		ec->ec_max_security = restrictinfo->security_level;
 		ec->ec_merged = NULL;
 		em1 = add_eq_member(ec, item1, item1_relids,
-							jdomain, NULL, item1_type);
+							jdomain, item1_type);
 		em2 = add_eq_member(ec, item2, item2_relids,
-							jdomain, NULL, item2_type);
+							jdomain, item2_type);
 
 		root->eq_classes = lappend(root->eq_classes, ec);
 
@@ -566,11 +584,13 @@ canonicalize_ec_expression(Expr *expr, Oid req_type, Oid req_collation)
 }
 
 /*
- * add_eq_member - build a new EquivalenceMember and add it to an EC
+ * make_eq_member
+ *		Build a new EquivalenceMember without adding it to an EC.  If 'parent'
+ *		is NULL, the result will be a parent member, otherwise a child member.
  */
 static EquivalenceMember *
-add_eq_member(EquivalenceClass *ec, Expr *expr, Relids relids,
+make_eq_member(EquivalenceClass *ec, Expr *expr, Relids relids,
-			  JoinDomain *jdomain, EquivalenceMember *parent, Oid datatype)
+			   JoinDomain *jdomain, EquivalenceMember *parent, Oid datatype)
 {
 	EquivalenceMember *em = makeNode(EquivalenceMember);
 
@@ -597,11 +617,85 @@ add_eq_member(EquivalenceClass *ec, Expr *expr, Relids relids,
 		ec->ec_has_const = true;
 		/* it can't affect ec_relids */
 	}
-	else if (!parent)			/* child members don't add to ec_relids */
+
+	return em;
+}
+
+/*
+ * add_eq_member - build a new non-child EquivalenceMember and add it to 'ec'.
+ */
+static EquivalenceMember *
+add_eq_member(EquivalenceClass *ec, Expr *expr, Relids relids,
+			  JoinDomain *jdomain, Oid datatype)
+{
+	EquivalenceMember *em = make_eq_member(ec, expr, relids, jdomain,
+										   NULL, datatype);
+
+	/* add to the members list */
+	ec->ec_members = lappend(ec->ec_members, em);
+
+	/* record the relids for parent members */
+	ec->ec_relids = bms_add_members(ec->ec_relids, relids);
+
+	return em;
+}
+
+/*
+ * add_child_eq_member
+ *		Create an em_is_child=true EquivalenceMember and add it to 'ec'.
+ *
+ * 'root' is the PlannerInfo that 'ec' belongs to.
+ * 'ec' is the EquivalenceClass to add the child member to.
+ * 'ec_index' the index of 'ec' within root->eq_classes, or -1 if maintaining
+ * the RelOptInfo.eclass_indexes isn't needed.
+ * 'expr' is the em_expr for the new member.
+ * 'relids' is the 'em_relids' for the new member.
+ * 'jdomain' is the 'em_jdomain' for the new member.
+ * 'parent_em' is the parent member of the child to create.
+ * 'datatype' is the em_datatype of the new member.
+ * 'child_relid' defines which element of ec_childmembers to add this member
+ * to.  This is generally a RELOPT_OTHER_MEMBER_REL, but for set operations
+ * can be a RELOPT_BASEREL representing the set-op children.
+ */
+static EquivalenceMember *
+add_child_eq_member(PlannerInfo *root, EquivalenceClass *ec, int ec_index,
+					Expr *expr, Relids relids, JoinDomain *jdomain,
+					EquivalenceMember *parent_em, Oid datatype,
+					Index child_relid)
+{
+	EquivalenceMember *em;
+
+	Assert(parent_em != NULL);
+
+	/*
+	 * Allocate the array to store child members; an array of Lists indexed by
+	 * relid, or expand the existing one, if necessary.
+	 */
+	if (unlikely(ec->ec_childmembers_size < root->simple_rel_array_size))
 	{
-		ec->ec_relids = bms_add_members(ec->ec_relids, relids);
+		if (ec->ec_childmembers == NULL)
+			ec->ec_childmembers = palloc0_array(List *, root->simple_rel_array_size);
+		else
+			ec->ec_childmembers = repalloc0_array(ec->ec_childmembers, List *,
+												  ec->ec_childmembers_size,
+												  root->simple_rel_array_size);
+
+		ec->ec_childmembers_size = root->simple_rel_array_size;
+	}
+
+	em = make_eq_member(ec, expr, relids, jdomain, parent_em, datatype);
+
+	/* add member to the ec_childmembers List for the given child_relid */
+	ec->ec_childmembers[child_relid] = lappend(ec->ec_childmembers[child_relid], em);
+
+	/* Record this EC index for the child rel */
+	if (ec_index >= 0)
+	{
+		RelOptInfo *child_rel = root->simple_rel_array[child_relid];
+
+		child_rel->eclass_indexes =
+			bms_add_member(child_rel->eclass_indexes, ec_index);
 	}
-	ec->ec_members = lappend(ec->ec_members, em);
 
 	return em;
 }
@@ -672,7 +766,8 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 	foreach(lc1, root->eq_classes)
 	{
 		EquivalenceClass *cur_ec = (EquivalenceClass *) lfirst(lc1);
-		ListCell   *lc2;
+		EquivalenceMemberIterator it;
+		EquivalenceMember *cur_em;
 
 		/*
 		 * Never match to a volatile EC, except when we are looking at another
@@ -687,10 +782,9 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 		if (!equal(opfamilies, cur_ec->ec_opfamilies))
 			continue;
 
-		foreach(lc2, cur_ec->ec_members)
+		setup_eclass_member_iterator(&it, cur_ec, rel);
+		while ((cur_em = eclass_member_iterator_next(&it)) != NULL)
 		{
-			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2);
-
 			/*
 			 * Ignore child members unless they match the request.
 			 */
@@ -725,7 +819,9 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 	newec = makeNode(EquivalenceClass);
 	newec->ec_opfamilies = list_copy(opfamilies);
 	newec->ec_collation = collation;
+	newec->ec_childmembers_size = 0;
 	newec->ec_members = NIL;
+	newec->ec_childmembers = NULL;
 	newec->ec_sources = NIL;
 	newec->ec_derives_list = NIL;
 	newec->ec_derives_hash = NULL;
@@ -747,7 +843,7 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 	expr_relids = pull_varnos(root, (Node *) expr);
 
 	newem = add_eq_member(newec, copyObject(expr), expr_relids,
-						  jdomain, NULL, opcintype);
+						  jdomain, opcintype);
 
 	/*
 	 * add_eq_member doesn't check for volatile functions, set-returning
@@ -821,15 +917,16 @@ find_ec_member_matching_expr(EquivalenceClass *ec,
 							 Expr *expr,
 							 Relids relids)
 {
-	ListCell   *lc;
+	EquivalenceMemberIterator it;
+	EquivalenceMember *em;
 
 	/* We ignore binary-compatible relabeling on both ends */
 	while (expr && IsA(expr, RelabelType))
 		expr = ((RelabelType *) expr)->arg;
 
-	foreach(lc, ec->ec_members)
+	setup_eclass_member_iterator(&it, ec, relids);
+	while ((em = eclass_member_iterator_next(&it)) != NULL)
 	{
-		EquivalenceMember *em = (EquivalenceMember *) lfirst(lc);
 		Expr	   *emexpr;
 
 		/*
@@ -898,7 +995,8 @@ find_computable_ec_member(PlannerInfo *root,
 						  bool require_parallel_safe)
 {
 	List	   *exprvars;
-	ListCell   *lc;
+	EquivalenceMemberIterator it;
+	EquivalenceMember *em;
 
 	/*
 	 * Pull out the Vars and quasi-Vars present in "exprs".  In the typical
@@ -912,9 +1010,9 @@ find_computable_ec_member(PlannerInfo *root,
 							   PVC_INCLUDE_PLACEHOLDERS |
 							   PVC_INCLUDE_CONVERTROWTYPES);
 
-	foreach(lc, ec->ec_members)
+	setup_eclass_member_iterator(&it, ec, relids);
+	while ((em = eclass_member_iterator_next(&it)) != NULL)
 	{
-		EquivalenceMember *em = (EquivalenceMember *) lfirst(lc);
 		List	   *emvars;
 		ListCell   *lc2;
 
@@ -1193,6 +1291,9 @@ generate_base_implied_equalities_const(PlannerInfo *root,
 		return;
 	}
 
+	/* We don't expect any children yet */
+	Assert(ec->ec_childmembers == NULL);
+
 	/*
 	 * Find the constant member to use.  We prefer an actual constant to
 	 * pseudo-constants (such as Params), because the constraint exclusion
@@ -1219,7 +1320,8 @@ generate_base_implied_equalities_const(PlannerInfo *root,
 		Oid			eq_op;
 		RestrictInfo *rinfo;
 
-		Assert(!cur_em->em_is_child);	/* no children yet */
+		/* Child members should not exist in ec_members */
+		Assert(!cur_em->em_is_child);
 		if (cur_em == const_em)
 			continue;
 		eq_op = select_equality_operator(ec,
@@ -1283,12 +1385,17 @@ generate_base_implied_equalities_no_const(PlannerInfo *root,
 	prev_ems = (EquivalenceMember **)
 		palloc0(root->simple_rel_array_size * sizeof(EquivalenceMember *));
 
+	/* We don't expect any children yet */
+	Assert(ec->ec_childmembers == NULL);
+
 	foreach(lc, ec->ec_members)
 	{
 		EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc);
 		int			relid;
 
-		Assert(!cur_em->em_is_child);	/* no children yet */
+		/* Child members should not exist in ec_members */
+		Assert(!cur_em->em_is_child);
+
 		if (!bms_get_singleton_member(cur_em->em_relids, &relid))
 			continue;
 		Assert(relid < root->simple_rel_array_size);
@@ -1621,7 +1728,8 @@ generate_join_implied_equalities_normal(PlannerInfo *root,
 	List	   *new_members = NIL;
 	List	   *outer_members = NIL;
 	List	   *inner_members = NIL;
-	ListCell   *lc1;
+	EquivalenceMemberIterator it;
+	EquivalenceMember *cur_em;
 
 	/*
 	 * First, scan the EC to identify member values that are computable at the
@@ -1632,10 +1740,9 @@ generate_join_implied_equalities_normal(PlannerInfo *root,
 	 * as well as to at least one input member, plus enforce at least one
 	 * outer-rel member equal to at least one inner-rel member.
 	 */
-	foreach(lc1, ec->ec_members)
+	setup_eclass_member_iterator(&it, ec, join_relids);
+	while ((cur_em = eclass_member_iterator_next(&it)) != NULL)
 	{
-		EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc1);
-
 		/*
 		 * We don't need to check explicitly for child EC members.  This test
 		 * against join_relids will cause them to be ignored except when
@@ -1668,6 +1775,7 @@ generate_join_implied_equalities_normal(PlannerInfo *root,
 		Oid			best_eq_op = InvalidOid;
 		int			best_score = -1;
 		RestrictInfo *rinfo;
+		ListCell   *lc1;
 
 		foreach(lc1, outer_members)
 		{
@@ -1742,6 +1850,7 @@ generate_join_implied_equalities_normal(PlannerInfo *root,
 		List	   *old_members = list_concat(outer_members, inner_members);
 		EquivalenceMember *prev_em = NULL;
 		RestrictInfo *rinfo;
+		ListCell   *lc1;
 
 		/* For now, arbitrarily take the first old_member as the one to use */
 		if (old_members)
@@ -1749,7 +1858,7 @@ generate_join_implied_equalities_normal(PlannerInfo *root,
 
 		foreach(lc1, new_members)
 		{
-			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc1);
+			cur_em = (EquivalenceMember *) lfirst(lc1);
 
 			if (prev_em != NULL)
 			{
@@ -2188,6 +2297,9 @@ reconsider_outer_join_clause(PlannerInfo *root, OuterJoinClauseInfo *ojcinfo,
 		bool		match;
 		ListCell   *lc2;
 
+		/* We don't expect any children yet */
+		Assert(cur_ec->ec_childmembers == NULL);
+
 		/* Ignore EC unless it contains pseudoconstants */
 		if (!cur_ec->ec_has_const)
 			continue;
@@ -2205,7 +2317,8 @@ reconsider_outer_join_clause(PlannerInfo *root, OuterJoinClauseInfo *ojcinfo,
 		{
 			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2);
 
-			Assert(!cur_em->em_is_child);	/* no children yet */
+			/* Child members should not exist in ec_members */
+			Assert(!cur_em->em_is_child);
 			if (equal(outervar, cur_em->em_expr))
 			{
 				match = true;
@@ -2303,6 +2416,9 @@ reconsider_full_join_clause(PlannerInfo *root, OuterJoinClauseInfo *ojcinfo)
 		ListCell   *lc2;
 		int			coal_idx = -1;
 
+		/* We don't expect any children yet */
+		Assert(cur_ec->ec_childmembers == NULL);
+
 		/* Ignore EC unless it contains pseudoconstants */
 		if (!cur_ec->ec_has_const)
 			continue;
@@ -2332,7 +2448,9 @@ reconsider_full_join_clause(PlannerInfo *root, OuterJoinClauseInfo *ojcinfo)
 		foreach(lc2, cur_ec->ec_members)
 		{
 			coal_em = (EquivalenceMember *) lfirst(lc2);
-			Assert(!coal_em->em_is_child);	/* no children yet */
+
+			/* Child members should not exist in ec_members */
+			Assert(!coal_em->em_is_child);
 			if (IsA(coal_em->em_expr, CoalesceExpr))
 			{
 				CoalesceExpr *cexpr = (CoalesceExpr *) coal_em->em_expr;
@@ -2461,6 +2579,13 @@ rebuild_eclass_attr_needed(PlannerInfo *root)
 	{
 		EquivalenceClass *ec = (EquivalenceClass *) lfirst(lc);
 
+		/*
+		 * We don't expect any EC child members to exist at this point. Ensure
+		 * that's the case, otherwise, we might be getting asked to do
+		 * something this function hasn't been coded for.
+		 */
+		Assert(ec->ec_childmembers == NULL);
+
 		/* Need do anything only for a multi-member, no-const EC. */
 		if (list_length(ec->ec_members) > 1 && !ec->ec_has_const)
 		{
@@ -2546,12 +2671,13 @@ exprs_known_equal(PlannerInfo *root, Node *item1, Node *item2, Oid opfamily)
 			!list_member_oid(ec->ec_opfamilies, opfamily))
 			continue;
 
+		/* Ignore children here */
 		foreach(lc2, ec->ec_members)
 		{
 			EquivalenceMember *em = (EquivalenceMember *) lfirst(lc2);
 
-			if (em->em_is_child)
+			/* Child members should not exist in ec_members */
-				continue;		/* ignore children here */
+			Assert(!em->em_is_child);
 			if (equal(item1, em->em_expr))
 				item1member = true;
 			else if (equal(item2, em->em_expr))
@@ -2615,15 +2741,18 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
 		/* Never match to a volatile EC */
 		if (ec->ec_has_volatile)
 			continue;
-		/* It's okay to consider "broken" ECs here, see exprs_known_equal */
 
+		/*
+		 * It's okay to consider "broken" ECs here, see exprs_known_equal.
+		 * Ignore children here.
+		 */
 		foreach(lc2, ec->ec_members)
 		{
 			EquivalenceMember *em = (EquivalenceMember *) lfirst(lc2);
 			Var		   *var;
 
-			if (em->em_is_child)
+			/* Child members should not exist in ec_members */
-				continue;		/* ignore children here */
+			Assert(!em->em_is_child);
 
 			/* EM must be a Var, possibly with RelabelType */
 			var = (Var *) em->em_expr;
@@ -2721,7 +2850,6 @@ add_child_rel_equivalences(PlannerInfo *root,
 	while ((i = bms_next_member(parent_rel->eclass_indexes, i)) >= 0)
 	{
 		EquivalenceClass *cur_ec = (EquivalenceClass *) list_nth(root->eq_classes, i);
-		int			num_members;
 
 		/*
 		 * If this EC contains a volatile expression, then generating child
@@ -2734,29 +2862,13 @@ add_child_rel_equivalences(PlannerInfo *root,
 		/* Sanity check eclass_indexes only contain ECs for parent_rel */
 		Assert(bms_is_subset(top_parent_relids, cur_ec->ec_relids));
 
-		/*
+		foreach_node(EquivalenceMember, cur_em, cur_ec->ec_members)
-		 * We don't use foreach() here because there's no point in scanning
-		 * newly-added child members, so we can stop after the last
-		 * pre-existing EC member.
-		 */
-		num_members = list_length(cur_ec->ec_members);
-		for (int pos = 0; pos < num_members; pos++)
 		{
-			EquivalenceMember *cur_em = (EquivalenceMember *) list_nth(cur_ec->ec_members, pos);
-
 			if (cur_em->em_is_const)
 				continue;		/* ignore consts here */
 
-			/*
+			/* Child members should not exist in ec_members */
-			 * We consider only original EC members here, not
+			Assert(!cur_em->em_is_child);
-			 * already-transformed child members.  Otherwise, if some original
-			 * member expression references more than one appendrel, we'd get
-			 * an O(N^2) explosion of useless derived expressions for
-			 * combinations of children.  (But add_child_join_rel_equivalences
-			 * may add targeted combinations for partitionwise-join purposes.)
-			 */
-			if (cur_em->em_is_child)
-				continue;		/* ignore children here */
 
 			/*
 			 * Consider only members that reference and can be computed at
@@ -2801,12 +2913,15 @@ add_child_rel_equivalences(PlannerInfo *root,
 											top_parent_relids);
 				new_relids = bms_add_members(new_relids, child_relids);
 
-				(void) add_eq_member(cur_ec, child_expr, new_relids,
+				add_child_eq_member(root,
-									 cur_em->em_jdomain,
+									cur_ec,
-									 cur_em, cur_em->em_datatype);
+									i,
-
+									child_expr,
-				/* Record this EC index for the child rel */
+									new_relids,
-				child_rel->eclass_indexes = bms_add_member(child_rel->eclass_indexes, i);
+									cur_em->em_jdomain,
+									cur_em,
+									cur_em->em_datatype,
+									child_rel->relid);
 			}
 		}
 	}
@@ -2853,7 +2968,6 @@ add_child_join_rel_equivalences(PlannerInfo *root,
 	while ((i = bms_next_member(matching_ecs, i)) >= 0)
 	{
 		EquivalenceClass *cur_ec = (EquivalenceClass *) list_nth(root->eq_classes, i);
-		int			num_members;
 
 		/*
 		 * If this EC contains a volatile expression, then generating child
@@ -2866,25 +2980,13 @@ add_child_join_rel_equivalences(PlannerInfo *root,
 		/* Sanity check on get_eclass_indexes_for_relids result */
 		Assert(bms_overlap(top_parent_relids, cur_ec->ec_relids));
 
-		/*
+		foreach_node(EquivalenceMember, cur_em, cur_ec->ec_members)
-		 * We don't use foreach() here because there's no point in scanning
-		 * newly-added child members, so we can stop after the last
-		 * pre-existing EC member.
-		 */
-		num_members = list_length(cur_ec->ec_members);
-		for (int pos = 0; pos < num_members; pos++)
 		{
-			EquivalenceMember *cur_em = (EquivalenceMember *) list_nth(cur_ec->ec_members, pos);
-
 			if (cur_em->em_is_const)
 				continue;		/* ignore consts here */
 
-			/*
+			/* Child members should not exist in ec_members */
-			 * We consider only original EC members here, not
+			Assert(!cur_em->em_is_child);
-			 * already-transformed child members.
-			 */
-			if (cur_em->em_is_child)
-				continue;		/* ignore children here */
 
 			/*
 			 * We may ignore expressions that reference a single baserel,
@@ -2929,9 +3031,35 @@ add_child_join_rel_equivalences(PlannerInfo *root,
 											top_parent_relids);
 				new_relids = bms_add_members(new_relids, child_relids);
 
-				(void) add_eq_member(cur_ec, child_expr, new_relids,
+				/*
-									 cur_em->em_jdomain,
+				 * Add new child member to the EquivalenceClass.  Because this
-									 cur_em, cur_em->em_datatype);
+				 * is a RELOPT_OTHER_JOINREL which has multiple component
+				 * relids, there is no ideal place to store these members in
+				 * the class.  Ordinarily, child members are stored in the
+				 * ec_childmembers[] array element corresponding to their
+				 * relid, however, here we have multiple component relids, so
+				 * there's no single ec_childmembers[] array element to store
+				 * this member.  So that we still correctly find this member
+				 * in loops iterating over an EquivalenceMemberIterator, we
+				 * opt to store the member in the ec_childmembers array in
+				 * only the first component relid slot of the array.  This
+				 * allows the member to be found, providing callers of
+				 * setup_eclass_member_iterator() specify all the component
+				 * relids for the RELOPT_OTHER_JOINREL, which they do.  If we
+				 * opted to store the member in each ec_childmembers[] element
+				 * for all the component relids, then that would just result
+				 * in eclass_member_iterator_next() finding the member
+				 * multiple times, which is a waste of effort.
+				 */
+				add_child_eq_member(root,
+									cur_ec,
+									-1,
+									child_expr,
+									new_relids,
+									cur_em->em_jdomain,
+									cur_em,
+									cur_em->em_datatype,
+									bms_next_member(child_joinrel->relids, -1));
 			}
 		}
 	}
@@ -2978,14 +3106,18 @@ add_setop_child_rel_equivalences(PlannerInfo *root, RelOptInfo *child_rel,
 		 * We can safely pass the parent member as the first member in the
 		 * ec_members list as this is added first in generate_union_paths,
 		 * likewise, the JoinDomain can be that of the initial member of the
-		 * Pathkey's EquivalenceClass.
+		 * Pathkey's EquivalenceClass.  We pass -1 for ec_index since we
+		 * maintain the eclass_indexes for the child_rel after the loop.
 		 */
-		add_eq_member(pk->pk_eclass,
+		add_child_eq_member(root,
-					  tle->expr,
+							pk->pk_eclass,
-					  child_rel->relids,
+							-1,
-					  parent_em->em_jdomain,
+							tle->expr,
-					  parent_em,
+							child_rel->relids,
-					  exprType((Node *) tle->expr));
+							parent_em->em_jdomain,
+							parent_em,
+							exprType((Node *) tle->expr),
+							child_rel->relid);
 
 		lc2 = lnext(setop_pathkeys, lc2);
 	}
@@ -3000,6 +3132,85 @@ add_setop_child_rel_equivalences(PlannerInfo *root, RelOptInfo *child_rel,
 											  list_length(root->eq_classes) - 1);
 }
 
+/*
+ * setup_eclass_member_iterator
+ *	  Setup an EquivalenceMemberIterator 'it' to iterate over all parent
+ *	  EquivalenceMembers and child members belonging to the given 'ec'.
+ *
+ * This iterator returns:
+ *	- All parent members stored directly in ec_members for 'ec', and;
+ *	- Any child member added to the given ec by add_child_eq_member() where
+ *	  the child_relid specified in the add_child_eq_member() call is a member
+ *	  of the 'child_relids' parameter.
+ *
+ * Note:
+ * The given 'child_relids' must remain allocated and not be changed for the
+ * lifetime of the iterator.
+ *
+ * Parameters:
+ *	'it' is a pointer to the iterator to set up.  Normally stack allocated.
+ *	'ec' is the EquivalenceClass from which to iterate members for.
+ *	'child_relids' is the relids to return child members for.
+ */
+void
+setup_eclass_member_iterator(EquivalenceMemberIterator *it,
+							 EquivalenceClass *ec, Relids child_relids)
+{
+	it->ec = ec;
+	/* no need to set this if the class has no child members array set */
+	it->child_relids = ec->ec_childmembers != NULL ? child_relids : NULL;
+	it->current_relid = -1;
+	it->current_list = ec->ec_members;
+	it->current_cell = list_head(it->current_list);
+}
+
+/*
+ * eclass_member_iterator_next
+ *	  Get the next EquivalenceMember from the EquivalenceMemberIterator 'it',
+ *	  as setup by setup_eclass_member_iterator().  NULL is returned if there
+ *	  are no members left, after which callers must not call
+ *	  eclass_member_iterator_next() again for the given iterator.
+ */
+EquivalenceMember *
+eclass_member_iterator_next(EquivalenceMemberIterator *it)
+{
+	while (it->current_list != NULL)
+	{
+		while (it->current_cell != NULL)
+		{
+			EquivalenceMember *em;
+
+	nextcell:
+			em = lfirst_node(EquivalenceMember, it->current_cell);
+			it->current_cell = lnext(it->current_list, it->current_cell);
+			return em;
+		}
+
+		/* Search for the next list to return members from */
+		while ((it->current_relid = bms_next_member(it->child_relids, it->current_relid)) > 0)
+		{
+			/*
+			 * Be paranoid in case we're given relids above what we've sized
+			 * the ec_childmembers array to.
+			 */
+			if (it->current_relid >= it->ec->ec_childmembers_size)
+				return NULL;
+
+			it->current_list = it->ec->ec_childmembers[it->current_relid];
+
+			/* If there are members in this list, use it. */
+			if (it->current_list != NIL)
+			{
+				/* point current_cell to the head of this list */
+				it->current_cell = list_head(it->current_list);
+				goto nextcell;
+			}
+		}
+		return NULL;
+	}
+
+	return NULL;
+}
 
 /*
  * generate_implied_equalities_for_column
@@ -3052,6 +3263,7 @@ generate_implied_equalities_for_column(PlannerInfo *root,
 	while ((i = bms_next_member(rel->eclass_indexes, i)) >= 0)
 	{
 		EquivalenceClass *cur_ec = (EquivalenceClass *) list_nth(root->eq_classes, i);
+		EquivalenceMemberIterator it;
 		EquivalenceMember *cur_em;
 		ListCell   *lc2;
 
@@ -3075,14 +3287,12 @@ generate_implied_equalities_for_column(PlannerInfo *root,
 		 * corner cases, so for now we live with just reporting the first
 		 * match.  See also get_eclass_for_sort_expr.)
 		 */
-		cur_em = NULL;
+		setup_eclass_member_iterator(&it, cur_ec, rel->relids);
-		foreach(lc2, cur_ec->ec_members)
+		while ((cur_em = eclass_member_iterator_next(&it)) != NULL)
 		{
-			cur_em = (EquivalenceMember *) lfirst(lc2);
 			if (bms_equal(cur_em->em_relids, rel->relids) &&
 				callback(root, rel, cur_ec, cur_em, callback_arg))
 				break;
-			cur_em = NULL;
 		}
 
 		if (!cur_em)
@@ -3090,7 +3300,7 @@ generate_implied_equalities_for_column(PlannerInfo *root,
 
 		/*
 		 * Found our match.  Scan the other EC members and attempt to generate
-		 * joinclauses.
+		 * joinclauses.  Ignore children here.
 		 */
 		foreach(lc2, cur_ec->ec_members)
 		{
@@ -3098,8 +3308,8 @@ generate_implied_equalities_for_column(PlannerInfo *root,
 			Oid			eq_op;
 			RestrictInfo *rinfo;
 
-			if (other_em->em_is_child)
+			/* Child members should not exist in ec_members */
-				continue;		/* ignore children here */
+			Assert(!other_em->em_is_child);
 
 			/* Make sure it'll be a join to a different rel */
 			if (other_em == cur_em ||
@@ -3312,13 +3522,15 @@ eclass_useful_for_merging(PlannerInfo *root,
 	if (bms_is_subset(eclass->ec_relids, relids))
 		return false;
 
-	/* To join, we need a member not in the given rel */
+	/*
+	 * To join, we need a member not in the given rel.  Ignore children here.
+	 */
 	foreach(lc, eclass->ec_members)
 	{
 		EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc);
 
-		if (cur_em->em_is_child)
+		/* Child members should not exist in ec_members */
-			continue;			/* ignore children here */
+		Assert(!cur_em->em_is_child);
 
 		if (!bms_overlap(cur_em->em_relids, relids))
 			return true;

src/backend/optimizer/path/indxpath.c

@@ -3755,7 +3755,8 @@ match_pathkeys_to_index(IndexOptInfo *index, List *pathkeys,
 	{
 		PathKey    *pathkey = (PathKey *) lfirst(lc1);
 		bool		found = false;
-		ListCell   *lc2;
+		EquivalenceMemberIterator it;
+		EquivalenceMember *member;
 
 
 		/* Pathkey must request default sort order for the target opfamily */
@@ -3774,9 +3775,10 @@ match_pathkeys_to_index(IndexOptInfo *index, List *pathkeys,
 		 * be considered to match more than one pathkey list, which is OK
 		 * here.  See also get_eclass_for_sort_expr.)
 		 */
-		foreach(lc2, pathkey->pk_eclass->ec_members)
+		setup_eclass_member_iterator(&it, pathkey->pk_eclass,
+									 index->rel->relids);
+		while ((member = eclass_member_iterator_next(&it)) != NULL)
 		{
-			EquivalenceMember *member = (EquivalenceMember *) lfirst(lc2);
 			int			indexcol;
 
 			/* No possibility of match if it references other relations */

src/backend/optimizer/path/pathkeys.c

@@ -1143,6 +1143,7 @@ convert_subquery_pathkeys(PlannerInfo *root, RelOptInfo *rel,
 			int			best_score = -1;
 			ListCell   *j;
 
+			/* Ignore children here */
 			foreach(j, sub_eclass->ec_members)
 			{
 				EquivalenceMember *sub_member = (EquivalenceMember *) lfirst(j);
@@ -1151,8 +1152,8 @@ convert_subquery_pathkeys(PlannerInfo *root, RelOptInfo *rel,
 				Oid			sub_expr_coll = sub_eclass->ec_collation;
 				ListCell   *k;
 
-				if (sub_member->em_is_child)
+				/* Child members should not exist in ec_members */
-					continue;	/* ignore children here */
+				Assert(!sub_member->em_is_child);
 
 				foreach(k, subquery_tlist)
 				{
@@ -1709,8 +1710,11 @@ select_outer_pathkeys_for_merge(PlannerInfo *root,
 		{
 			EquivalenceMember *em = (EquivalenceMember *) lfirst(lc2);
 
+			/* Child members should not exist in ec_members */
+			Assert(!em->em_is_child);
+
 			/* Potential future join partner? */
-			if (!em->em_is_const && !em->em_is_child &&
+			if (!em->em_is_const &&
 				!bms_overlap(em->em_relids, joinrel->relids))
 				score++;
 		}

src/backend/optimizer/plan/analyzejoins.c

@@ -710,6 +710,13 @@ remove_rel_from_eclass(EquivalenceClass *ec, SpecialJoinInfo *sjinfo,
 		ec->ec_relids = adjust_relid_set(ec->ec_relids,
 										 sjinfo->ojrelid, subst);
 
+	/*
+	 * We don't expect any EC child members to exist at this point.  Ensure
+	 * that's the case, otherwise, we might be getting asked to do something
+	 * this function hasn't been coded for.
+	 */
+	Assert(ec->ec_childmembers == NULL);
+
 	/*
 	 * Fix up the member expressions.  Any non-const member that ends with
 	 * empty em_relids must be a Var or PHV of the removed relation.  We don't
@@ -1509,6 +1516,13 @@ update_eclasses(EquivalenceClass *ec, int from, int to)
 	List	   *new_members = NIL;
 	List	   *new_sources = NIL;
 
+	/*
+	 * We don't expect any EC child members to exist at this point.  Ensure
+	 * that's the case, otherwise, we might be getting asked to do something
+	 * this function hasn't been coded for.
+	 */
+	Assert(ec->ec_childmembers == NULL);
+
 	foreach_node(EquivalenceMember, em, ec->ec_members)
 	{
 		bool		is_redundant = false;

src/include/nodes/pathnodes.h

@@ -1414,6 +1414,24 @@ typedef struct JoinDomain
  * In contrast, ec_sources holds equality clauses that appear directly in the
  * query. These are typically few and do not require a hash table for lookup.
  *
+ * 'ec_members' is a List of all !em_is_child EquivalenceMembers in the class.
+ * EquivalenceMembers for any RELOPT_OTHER_MEMBER_REL and RELOPT_OTHER_JOINREL
+ * relations are stored in the 'ec_childmembers' array in the index
+ * corresponding to the relid, or first component relid in the case of
+ * RELOPT_OTHER_JOINRELs.  'ec_childmembers' is NULL if the class has no child
+ * EquivalenceMembers.
+ *
+ * For code wishing to look at EquivalenceMembers, if only parent-level
+ * members are needed, then a simple foreach loop over ec_members is
+ * sufficient.  When child members are also required, it is best to use the
+ * functionality provided by EquivalenceMemberIterator.  This visits all
+ * parent members and only the relevant child members.  The reason for this
+ * is that large numbers of child EquivalenceMembers can exist in queries to
+ * partitioned tables with many partitions.  The functionality provided by
+ * EquivalenceMemberIterator allows efficient access to EquivalenceMembers
+ * which belong to specific child relids.  See the header comments for
+ * EquivalenceMemberIterator below for further details.
+ *
  * NB: if ec_merged isn't NULL, this class has been merged into another, and
  * should be ignored in favor of using the pointed-to class.
  *
@@ -1431,7 +1449,9 @@ typedef struct EquivalenceClass
 
 	List	   *ec_opfamilies;	/* btree operator family OIDs */
 	Oid			ec_collation;	/* collation, if datatypes are collatable */
+	int			ec_childmembers_size;	/* # elements in ec_childmembers */
 	List	   *ec_members;		/* list of EquivalenceMembers */
+	List	  **ec_childmembers;	/* array of Lists of child members */
 	List	   *ec_sources;		/* list of generating RestrictInfos */
 	List	   *ec_derives_list;	/* list of derived RestrictInfos */
 	struct derives_hash *ec_derives_hash;	/* optional hash table for fast
@@ -1465,12 +1485,17 @@ typedef struct EquivalenceClass
  * child when necessary to build a MergeAppend path for the whole appendrel
  * tree.  An em_is_child member has no impact on the properties of the EC as a
  * whole; in particular the EC's ec_relids field does NOT include the child
- * relation.  An em_is_child member should never be marked em_is_const nor
+ * relation.  em_is_child members aren't stored in the ec_members List of the
- * cause ec_has_const or ec_has_volatile to be set, either.  Thus, em_is_child
+ * EC and instead they're stored and indexed by the relids of the child
- * members are not really full-fledged members of the EC, but just reflections
+ * relation they represent in ec_childmembers.  An em_is_child member
- * or doppelgangers of real members.  Most operations on EquivalenceClasses
+ * should never be marked em_is_const nor cause ec_has_const or
- * should ignore em_is_child members, and those that don't should test
+ * ec_has_volatile to be set, either.  Thus, em_is_child members are not
- * em_relids to make sure they only consider relevant members.
+ * really full-fledged members of the EC, but just reflections or
+ * doppelgangers of real members.  Most operations on EquivalenceClasses
+ * should ignore em_is_child members by only inspecting members in the
+ * ec_members list.  Callers that require inspecting child members should do
+ * so using an EquivalenceMemberIterator and should test em_relids to make
+ * sure they only consider relevant members.
  *
  * em_datatype is usually the same as exprType(em_expr), but can be
  * different when dealing with a binary-compatible opfamily; in particular
@@ -1493,6 +1518,67 @@ typedef struct EquivalenceMember
 	struct EquivalenceMember *em_parent pg_node_attr(read_write_ignore);
 } EquivalenceMember;
 
+/*
+ * EquivalenceMemberIterator
+ *
+ * EquivalenceMemberIterator allows efficient access to sets of
+ * EquivalenceMembers for callers which require access to child members.
+ * Because partitioning workloads can result in large numbers of child
+ * members, the child members are not stored in the EquivalenceClass's
+ * ec_members List.  Instead, these are stored in the EquivalenceClass's
+ * ec_childmembers array of Lists.  The functionality provided by
+ * EquivalenceMemberIterator aims to provide efficient access to parent
+ * members and child members belonging to specific child relids.
+ *
+ * Currently, there is only one way to initialize and iterate over an
+ * EquivalenceMemberIterator and that is via the setup_eclass_member_iterator
+ * and eclass_member_iterator_next functions.  The iterator object is
+ * generally a local variable which is passed by address to
+ * setup_eclass_member_iterator.  The calling function defines which
+ * EquivalenceClass the iterator should be looking at and which child
+ * relids to also return members for.  child_relids can be passed as NULL, but
+ * the caller may as well just perform a foreach loop over ec_members as only
+ * parent-level members will be returned in that case.
+ *
+ * When calling the next function on an EquivalenceMemberIterator, all
+ * parent-level EquivalenceMembers are returned first, followed by all child
+ * members for the specified 'child_relids' for all child members which were
+ * indexed by any of the specified 'child_relids' in add_child_eq_member().
+ *
+ * Code using the iterator method of finding EquivalenceMembers will generally
+ * always want to ensure the returned member matches their search criteria
+ * rather than relying on the filtering to be done for them as all parent
+ * members are returned and for members belonging to RELOPT_OTHER_JOINREL
+ * rels, the member's em_relids may be a superset of the specified
+ * 'child_relids', which might not be what the caller wants.
+ *
+ * The most common way to use this iterator is as follows:
+ * -----
+ * EquivalenceMemberIterator		it;
+ * EquivalenceMember			   *em;
+ *
+ * setup_eclass_member_iterator(&it, ec, child_relids);
+ * while ((em = eclass_member_iterator_next(&it)) != NULL)
+ * {
+ *		...
+ * }
+ * -----
+ * It is not valid to call eclass_member_iterator_next() after it has returned
+ * NULL for any given EquivalenceMemberIterator.  Individual fields within
+ * the EquivalenceMemberIterator struct must not be accessed by callers.
+ */
+typedef struct
+{
+	EquivalenceClass *ec;		/* The EquivalenceClass to iterate over */
+	int			current_relid;	/* Current relid position within 'relids'. -1
+								 * when still looping over ec_members and -2
+								 * at the end of iteration */
+	Relids		child_relids;	/* Relids of child relations of interest.
+								 * Non-child rels are ignored */
+	ListCell   *current_cell;	/* Next cell to return within current_list */
+	List	   *current_list;	/* Current list of members being returned */
+} EquivalenceMemberIterator;
+
 /*
  * PathKeys
  *

src/include/optimizer/paths.h

@@ -183,6 +183,10 @@ extern void add_setop_child_rel_equivalences(PlannerInfo *root,
 											 RelOptInfo *child_rel,
 											 List *child_tlist,
 											 List *setop_pathkeys);
+extern void setup_eclass_member_iterator(EquivalenceMemberIterator *it,
+										 EquivalenceClass *ec,
+										 Relids child_relids);
+extern EquivalenceMember *eclass_member_iterator_next(EquivalenceMemberIterator *it);
 extern List *generate_implied_equalities_for_column(PlannerInfo *root,
 													RelOptInfo *rel,
 													ec_matches_callback_type callback,

src/tools/pgindent/typedefs.list

@@ -714,6 +714,7 @@ EphemeralNamedRelationMetadata
 EphemeralNamedRelationMetadataData
 EquivalenceClass
 EquivalenceMember
+EquivalenceMemberIterator
 ErrorContextCallback
 ErrorData
 ErrorSaveContext