Remove useless self-joins

The Self Join Elimination (SJE) feature removes an inner join of a plain table
to itself in the query tree if is proved that the join can be replaced with
a scan without impacting the query result.  Self join and inner relation are
replaced with the outer in query, equivalence classes, and planner info
structures. Also, inner restrictlist moves to the outer one with removing
duplicated clauses. Thus, this optimization reduces the length of the range
table list (this especially makes sense for partitioned relations), reduces
the number of restriction clauses === selectivity estimations, and potentially
can improve total planner prediction for the query.

The SJE proof is based on innerrel_is_unique machinery.

We can remove a self-join when for each outer row:
 1. At most one inner row matches the join clause.
 2. Each matched inner row must be (physically) the same row as the outer one.

In this patch we use the next approach to identify a self-join:
 1. Collect all merge-joinable join quals which look like a.x = b.x
 2. Add to the list above the baseretrictinfo of the inner table.
 3. Check innerrel_is_unique() for the qual list.  If it returns false, skip
    this pair of joining tables.
 4. Check uniqueness, proved by the baserestrictinfo clauses. To prove
    the possibility of self-join elimination inner and outer clauses must have
    an exact match.

The relation replacement procedure is not trivial and it is partly combined
with the one, used to remove useless left joins.  Tests, covering this feature,
were added to join.sql.  Some regression tests changed due to self-join removal
logic.

Discussion: https://postgr.es/m/flat/64486b0b-0404-e39e-322d-0801154901f3%40postgrespro.ru
Author: Andrey Lepikhov, Alexander Kuzmenkov
Reviewed-by: Tom Lane, Robert Haas, Andres Freund, Simon Riggs, Jonathan S. Katz
Reviewed-by: David Rowley, Thomas Munro, Konstantin Knizhnik, Heikki Linnakangas
Reviewed-by: Hywel Carver, Laurenz Albe, Ronan Dunklau, vignesh C, Zhihong Yu
Reviewed-by: Greg Stark, Jaime Casanova, Michał Kłeczek, Alena Rybakina
Reviewed-by: Alexander Korotkov

1 files changed, 1154 insertions, 57 deletions

diff --git a/src/backend/optimizer/plan/analyzejoins.c b/src/backend/optimizer/plan/analyzejoins.c
index 5f3cce873a0..2e14268e188 100644
--- a/src/backend/optimizer/plan/analyzejoins.c
+++ b/src/backend/optimizer/plan/analyzejoins.c
@@ -22,6 +22,7 @@
  */
 #include "postgres.h"
 
+#include "catalog/pg_class.h"
 #include "nodes/nodeFuncs.h"
 #include "optimizer/clauses.h"
 #include "optimizer/joininfo.h"
@@ -33,10 +34,45 @@
 #include "optimizer/tlist.h"
 #include "utils/lsyscache.h"
 
+/*
+ * UniqueRelInfo caches a fact that a relation is unique when being joined
+ * to other relation(s) specified by outerrelids.
+ * 'extra_clauses' contains additional clauses from a baserestrictinfo list that
+ * were used to prove uniqueness. We cache it for the SJ checking procedure: SJ
+ * can be removed if the outer relation contains strictly the same set of
+ * clauses.
+ */
+typedef struct UniqueRelInfo
+{
+	Relids		outerrelids;
+	List	   *extra_clauses;
+} UniqueRelInfo;
+
+/*
+ * The context for replace_varno_walker() containing source and target relids.
+ */
+typedef struct
+{
+	int			from;
+	int			to;
+} ReplaceVarnoContext;
+
+/*
+ * The struct containing self-join candidate.  Used to find duplicate reloids.
+ */
+typedef struct
+{
+	int			relid;
+	Oid			reloid;
+} SelfJoinCandidate;
+
+bool		enable_self_join_removal;
+
 /* local functions */
 static bool join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo);
-static void remove_rel_from_query(PlannerInfo *root, int relid,
-								  SpecialJoinInfo *sjinfo);
+
+static void remove_leftjoinrel_from_query(PlannerInfo *root, int relid,
+										  SpecialJoinInfo *sjinfo);
 static void remove_rel_from_restrictinfo(RestrictInfo *rinfo,
 										 int relid, int ojrelid);
 static void remove_rel_from_eclass(EquivalenceClass *ec,
@@ -44,14 +80,20 @@ static void remove_rel_from_eclass(EquivalenceClass *ec,
 static List *remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved);
 static bool rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel);
 static bool rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel,
-								List *clause_list);
+								List *clause_list, List **extra_clauses);
 static Oid	distinct_col_search(int colno, List *colnos, List *opids);
 static bool is_innerrel_unique_for(PlannerInfo *root,
 								   Relids joinrelids,
 								   Relids outerrelids,
 								   RelOptInfo *innerrel,
 								   JoinType jointype,
-								   List *restrictlist);
+								   List *restrictlist,
+								   List **extra_clauses);
+static Bitmapset *replace_relid(Relids relids, int oldId, int newId);
+static void replace_varno(Node *node, int from, int to);
+static bool replace_varno_walker(Node *node, ReplaceVarnoContext *ctx);
+static int	self_join_candidates_cmp(const void *a, const void *b);
+
 
 
 /*
@@ -89,7 +131,7 @@ restart:
 		 */
 		innerrelid = bms_singleton_member(sjinfo->min_righthand);
 
-		remove_rel_from_query(root, innerrelid, sjinfo);
+		remove_leftjoinrel_from_query(root, innerrelid, sjinfo);
 
 		/* We verify that exactly one reference gets removed from joinlist */
 		nremoved = 0;
@@ -308,7 +350,7 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 	 * Now that we have the relevant equality join clauses, try to prove the
 	 * innerrel distinct.
 	 */
-	if (rel_is_distinct_for(root, innerrel, clause_list))
+	if (rel_is_distinct_for(root, innerrel, clause_list, NULL))
 		return true;
 
 	/*
@@ -320,29 +362,24 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 
 
 /*
- * Remove the target relid and references to the target join from the
+ * Remove the target rel->relid and references to the target join from the
  * planner's data structures, having determined that there is no need
- * to include them in the query.
+ * to include them in the query.  Optionally replace them with subst if subst
+ * is non-negative.
  *
- * We are not terribly thorough here.  We only bother to update parts of
- * the planner's data structures that will actually be consulted later.
+ * This function updates only parts needed for both left-join removal and
+ * self-join removal.
  */
 static void
-remove_rel_from_query(PlannerInfo *root, int relid, SpecialJoinInfo *sjinfo)
+remove_rel_from_query(PlannerInfo *root, RelOptInfo *rel,
+					  int subst, SpecialJoinInfo *sjinfo,
+					  Relids joinrelids)
 {
-	RelOptInfo *rel = find_base_rel(root, relid);
-	int			ojrelid = sjinfo->ojrelid;
-	Relids		joinrelids;
-	Relids		join_plus_commute;
-	List	   *joininfos;
+	int			relid = rel->relid;
+	int			ojrelid = (sjinfo != NULL) ? sjinfo->ojrelid : -1;
 	Index		rti;
 	ListCell   *l;
 
-	/* Compute the relid set for the join we are considering */
-	joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
-	Assert(ojrelid != 0);
-	joinrelids = bms_add_member(joinrelids, ojrelid);
-
 	/*
 	 * Remove references to the rel from other baserels' attr_needed arrays.
 	 */
@@ -366,19 +403,22 @@ remove_rel_from_query(PlannerInfo *root, int relid, SpecialJoinInfo *sjinfo)
 			 attroff--)
 		{
 			otherrel->attr_needed[attroff] =
-				bms_del_member(otherrel->attr_needed[attroff], relid);
+				replace_relid(otherrel->attr_needed[attroff], relid, subst);
 			otherrel->attr_needed[attroff] =
-				bms_del_member(otherrel->attr_needed[attroff], ojrelid);
+				replace_relid(otherrel->attr_needed[attroff], ojrelid, subst);
 		}
+
+		/* Update lateral references. */
+		replace_varno((Node *) otherrel->lateral_vars, relid, subst);
 	}
 
 	/*
 	 * Update all_baserels and related relid sets.
 	 */
-	root->all_baserels = bms_del_member(root->all_baserels, relid);
-	root->outer_join_rels = bms_del_member(root->outer_join_rels, ojrelid);
-	root->all_query_rels = bms_del_member(root->all_query_rels, relid);
-	root->all_query_rels = bms_del_member(root->all_query_rels, ojrelid);
+	root->all_baserels = replace_relid(root->all_baserels, relid, subst);
+	root->outer_join_rels = replace_relid(root->outer_join_rels, ojrelid, subst);
+	root->all_query_rels = replace_relid(root->all_query_rels, relid, subst);
+	root->all_query_rels = replace_relid(root->all_query_rels, ojrelid, subst);
 
 	/*
 	 * Likewise remove references from SpecialJoinInfo data structures.
@@ -392,19 +432,21 @@ remove_rel_from_query(PlannerInfo *root, int relid, SpecialJoinInfo *sjinfo)
 	{
 		SpecialJoinInfo *sjinf = (SpecialJoinInfo *) lfirst(l);
 
-		sjinf->min_lefthand = bms_del_member(sjinf->min_lefthand, relid);
-		sjinf->min_righthand = bms_del_member(sjinf->min_righthand, relid);
-		sjinf->syn_lefthand = bms_del_member(sjinf->syn_lefthand, relid);
-		sjinf->syn_righthand = bms_del_member(sjinf->syn_righthand, relid);
-		sjinf->min_lefthand = bms_del_member(sjinf->min_lefthand, ojrelid);
-		sjinf->min_righthand = bms_del_member(sjinf->min_righthand, ojrelid);
-		sjinf->syn_lefthand = bms_del_member(sjinf->syn_lefthand, ojrelid);
-		sjinf->syn_righthand = bms_del_member(sjinf->syn_righthand, ojrelid);
+		sjinf->min_lefthand = replace_relid(sjinf->min_lefthand, relid, subst);
+		sjinf->min_righthand = replace_relid(sjinf->min_righthand, relid, subst);
+		sjinf->syn_lefthand = replace_relid(sjinf->syn_lefthand, relid, subst);
+		sjinf->syn_righthand = replace_relid(sjinf->syn_righthand, relid, subst);
+		sjinf->min_lefthand = replace_relid(sjinf->min_lefthand, ojrelid, subst);
+		sjinf->min_righthand = replace_relid(sjinf->min_righthand, ojrelid, subst);
+		sjinf->syn_lefthand = replace_relid(sjinf->syn_lefthand, ojrelid, subst);
+		sjinf->syn_righthand = replace_relid(sjinf->syn_righthand, ojrelid, subst);
 		/* relid cannot appear in these fields, but ojrelid can: */
-		sjinf->commute_above_l = bms_del_member(sjinf->commute_above_l, ojrelid);
-		sjinf->commute_above_r = bms_del_member(sjinf->commute_above_r, ojrelid);
-		sjinf->commute_below_l = bms_del_member(sjinf->commute_below_l, ojrelid);
-		sjinf->commute_below_r = bms_del_member(sjinf->commute_below_r, ojrelid);
+		sjinf->commute_above_l = replace_relid(sjinf->commute_above_l, ojrelid, subst);
+		sjinf->commute_above_r = replace_relid(sjinf->commute_above_r, ojrelid, subst);
+		sjinf->commute_below_l = replace_relid(sjinf->commute_below_l, ojrelid, subst);
+		sjinf->commute_below_r = replace_relid(sjinf->commute_below_r, ojrelid, subst);
+
+		replace_varno((Node *) sjinf->semi_rhs_exprs, relid, subst);
 	}
 
 	/*
@@ -438,18 +480,47 @@ remove_rel_from_query(PlannerInfo *root, int relid, SpecialJoinInfo *sjinfo)
 		{
 			PlaceHolderVar *phv = phinfo->ph_var;
 
-			phinfo->ph_eval_at = bms_del_member(phinfo->ph_eval_at, relid);
-			phinfo->ph_eval_at = bms_del_member(phinfo->ph_eval_at, ojrelid);
+			phinfo->ph_eval_at = replace_relid(phinfo->ph_eval_at, relid, subst);
+			phinfo->ph_eval_at = replace_relid(phinfo->ph_eval_at, ojrelid, subst);
 			Assert(!bms_is_empty(phinfo->ph_eval_at));	/* checked previously */
-			phinfo->ph_needed = bms_del_member(phinfo->ph_needed, relid);
-			phinfo->ph_needed = bms_del_member(phinfo->ph_needed, ojrelid);
+			phinfo->ph_needed = replace_relid(phinfo->ph_needed, relid, subst);
+			phinfo->ph_needed = replace_relid(phinfo->ph_needed, ojrelid, subst);
 			/* ph_needed might or might not become empty */
-			phv->phrels = bms_del_member(phv->phrels, relid);
-			phv->phrels = bms_del_member(phv->phrels, ojrelid);
+			phv->phrels = replace_relid(phv->phrels, relid, subst);
+			phv->phrels = replace_relid(phv->phrels, ojrelid, subst);
+			phinfo->ph_lateral = replace_relid(phinfo->ph_lateral, relid, subst);
+			phinfo->ph_var->phrels = replace_relid(phinfo->ph_var->phrels, relid, subst);
 			Assert(!bms_is_empty(phv->phrels));
 			Assert(phv->phnullingrels == NULL); /* no need to adjust */
 		}
 	}
+}
+
+/*
+ * Remove the target relid and references to the target join from the
+ * planner's data structures, having determined that there is no need
+ * to include them in the query.
+ *
+ * We are not terribly thorough here.  We only bother to update parts of
+ * the planner's data structures that will actually be consulted later.
+ */
+static void
+remove_leftjoinrel_from_query(PlannerInfo *root, int relid,
+							  SpecialJoinInfo *sjinfo)
+{
+	List	   *joininfos;
+	int			ojrelid = sjinfo->ojrelid;
+	RelOptInfo *rel = find_base_rel(root, relid);
+	Relids		join_plus_commute;
+	Relids		joinrelids;
+	ListCell   *l;
+
+	/* Compute the relid set for the join we are considering */
+	joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
+	Assert(ojrelid != 0);
+	joinrelids = bms_add_member(joinrelids, ojrelid);
+
+	remove_rel_from_query(root, rel, -1, sjinfo, joinrelids);
 
 	/*
 	 * Remove any joinquals referencing the rel from the joininfo lists.
@@ -844,9 +915,14 @@ rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel)
  * Note that the passed-in clause_list may be destructively modified!  This
  * is OK for current uses, because the clause_list is built by the caller for
  * the sole purpose of passing to this function.
+ *
+ * outer_exprs contains the right sides of baserestrictinfo clauses looking
+ * like x = const if distinctness is derived from such clauses, not joininfo
+ * clause.  Pass NULL to the outer_exprs, if its value is not needed.
  */
 static bool
-rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel, List *clause_list)
+rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel, List *clause_list,
+					List **extra_clauses)
 {
 	/*
 	 * We could skip a couple of tests here if we assume all callers checked
@@ -859,10 +935,11 @@ rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel, List *clause_list)
 	{
 		/*
 		 * Examine the indexes to see if we have a matching unique index.
-		 * relation_has_unique_index_for automatically adds any usable
+		 * relation_has_unique_index_ext automatically adds any usable
 		 * restriction clauses for the rel, so we needn't do that here.
 		 */
-		if (relation_has_unique_index_for(root, rel, clause_list, NIL, NIL))
+		if (relation_has_unique_index_ext(root, rel, clause_list, NIL, NIL,
+										  extra_clauses))
 			return true;
 	}
 	else if (rel->rtekind == RTE_SUBQUERY)
@@ -1177,8 +1254,29 @@ innerrel_is_unique(PlannerInfo *root,
 				   List *restrictlist,
 				   bool force_cache)
 {
+	return innerrel_is_unique_ext(root, joinrelids, outerrelids, innerrel,
+								  jointype, restrictlist, force_cache, NULL);
+}
+
+/*
+ * innerrel_is_unique_ext
+ *	  Do the same as innerrel_is_unique(), but also return additional clauses
+ *	  from a baserestrictinfo list that were used to prove uniqueness.
+ */
+bool
+innerrel_is_unique_ext(PlannerInfo *root,
+					   Relids joinrelids,
+					   Relids outerrelids,
+					   RelOptInfo *innerrel,
+					   JoinType jointype,
+					   List *restrictlist,
+					   bool force_cache,
+					   List **extra_clauses)
+{
 	MemoryContext old_context;
 	ListCell   *lc;
+	UniqueRelInfo *uniqueRelInfo;
+	List	   *outer_exprs = NIL;
 
 	/* Certainly can't prove uniqueness when there are no joinclauses */
 	if (restrictlist == NIL)
@@ -1200,10 +1298,14 @@ innerrel_is_unique(PlannerInfo *root,
 	 */
 	foreach(lc, innerrel->unique_for_rels)
 	{
-		Relids		unique_for_rels = (Relids) lfirst(lc);
+		uniqueRelInfo = (UniqueRelInfo *) lfirst(lc);
 
-		if (bms_is_subset(unique_for_rels, outerrelids))
+		if (bms_is_subset(uniqueRelInfo->outerrelids, outerrelids))
+		{
+			if (extra_clauses)
+				*extra_clauses = uniqueRelInfo->extra_clauses;
 			return true;		/* Success! */
+		}
 	}
 
 	/*
@@ -1220,7 +1322,7 @@ innerrel_is_unique(PlannerInfo *root,
 
 	/* No cached information, so try to make the proof. */
 	if (is_innerrel_unique_for(root, joinrelids, outerrelids, innerrel,
-							   jointype, restrictlist))
+							   jointype, restrictlist, &outer_exprs))
 	{
 		/*
 		 * Cache the positive result for future probes, being sure to keep it
@@ -1233,10 +1335,15 @@ innerrel_is_unique(PlannerInfo *root,
 		 * supersets of them anyway.
 		 */
 		old_context = MemoryContextSwitchTo(root->planner_cxt);
+		uniqueRelInfo = palloc(sizeof(UniqueRelInfo));
+		uniqueRelInfo->extra_clauses = outer_exprs;
+		uniqueRelInfo->outerrelids = bms_copy(outerrelids);
 		innerrel->unique_for_rels = lappend(innerrel->unique_for_rels,
-											bms_copy(outerrelids));
+											uniqueRelInfo);
 		MemoryContextSwitchTo(old_context);
 
+		if (extra_clauses)
+			*extra_clauses = outer_exprs;
 		return true;			/* Success! */
 	}
 	else
@@ -1282,7 +1389,8 @@ is_innerrel_unique_for(PlannerInfo *root,
 					   Relids outerrelids,
 					   RelOptInfo *innerrel,
 					   JoinType jointype,
-					   List *restrictlist)
+					   List *restrictlist,
+					   List **extra_clauses)
 {
 	List	   *clause_list = NIL;
 	ListCell   *lc;
@@ -1312,17 +1420,1006 @@ is_innerrel_unique_for(PlannerInfo *root,
 			continue;			/* not mergejoinable */
 
 		/*
-		 * Check if clause has the form "outer op inner" or "inner op outer",
-		 * and if so mark which side is inner.
+		 * Check if the clause has the form "outer op inner" or "inner op
+		 * outer", and if so mark which side is inner.
 		 */
 		if (!clause_sides_match_join(restrictinfo, outerrelids,
 									 innerrel->relids))
 			continue;			/* no good for these input relations */
 
-		/* OK, add to list */
+		/* OK, add to the list */
 		clause_list = lappend(clause_list, restrictinfo);
 	}
 
 	/* Let rel_is_distinct_for() do the hard work */
-	return rel_is_distinct_for(root, innerrel, clause_list);
+	return rel_is_distinct_for(root, innerrel, clause_list, extra_clauses);
+}
+
+/*
+ * Replace each occurrence of removing relid with the keeping one
+ */
+static void
+replace_varno(Node *node, int from, int to)
+{
+	ReplaceVarnoContext ctx;
+
+	if (to <= 0)
+		return;
+
+	ctx.from = from;
+	ctx.to = to;
+	replace_varno_walker(node, &ctx);
+}
+
+/*
+ * Walker function for replace_varno()
+ */
+static bool
+replace_varno_walker(Node *node, ReplaceVarnoContext *ctx)
+{
+	if (node == NULL)
+		return false;
+
+	if (IsA(node, Var))
+	{
+		Var		   *var = (Var *) node;
+
+		if (var->varno == ctx->from)
+		{
+			var->varno = ctx->to;
+			var->varnosyn = ctx->to;
+		}
+		return false;
+	}
+	if (IsA(node, RestrictInfo))
+	{
+		RestrictInfo *rinfo = (RestrictInfo *) node;
+		int			relid = -1;
+		bool		is_req_equal =
+			(rinfo->required_relids == rinfo->clause_relids);
+
+		if (bms_is_member(ctx->from, rinfo->clause_relids))
+		{
+			replace_varno((Node *) rinfo->clause, ctx->from, ctx->to);
+			replace_varno((Node *) rinfo->orclause, ctx->from, ctx->to);
+			rinfo->clause_relids = replace_relid(rinfo->clause_relids, ctx->from, ctx->to);
+			rinfo->left_relids = replace_relid(rinfo->left_relids, ctx->from, ctx->to);
+			rinfo->right_relids = replace_relid(rinfo->right_relids, ctx->from, ctx->to);
+		}
+
+		if (is_req_equal)
+			rinfo->required_relids = rinfo->clause_relids;
+		else
+			rinfo->required_relids = replace_relid(rinfo->required_relids, ctx->from, ctx->to);
+
+		rinfo->outer_relids = replace_relid(rinfo->outer_relids, ctx->from, ctx->to);
+		rinfo->incompatible_relids = replace_relid(rinfo->incompatible_relids, ctx->from, ctx->to);
+
+		if (rinfo->mergeopfamilies &&
+			bms_get_singleton_member(rinfo->clause_relids, &relid) &&
+			relid == ctx->to && IsA(rinfo->clause, OpExpr))
+		{
+			Expr	   *leftOp;
+			Expr	   *rightOp;
+
+			leftOp = (Expr *) get_leftop(rinfo->clause);
+			rightOp = (Expr *) get_rightop(rinfo->clause);
+
+			if (leftOp != NULL && equal(leftOp, rightOp))
+			{
+				NullTest   *ntest = makeNode(NullTest);
+
+				ntest->arg = leftOp;
+				ntest->nulltesttype = IS_NOT_NULL;
+				ntest->argisrow = false;
+				ntest->location = -1;
+				rinfo->clause = (Expr *) ntest;
+				rinfo->mergeopfamilies = NIL;
+			}
+			Assert(rinfo->orclause == NULL);
+		}
+
+		return false;
+	}
+	return expression_tree_walker(node, replace_varno_walker, (void *) ctx);
+}
+
+/*
+ * Substitute newId by oldId in relids.
+ */
+static Bitmapset *
+replace_relid(Relids relids, int oldId, int newId)
+{
+	if (oldId < 0)
+		return relids;
+
+	if (newId < 0)
+		/* Delete relid without substitution. */
+		return bms_del_member(relids, oldId);
+
+	if (bms_is_member(oldId, relids))
+		return bms_add_member(bms_del_member(relids, oldId), newId);
+
+	return relids;
+}
+
+/*
+ * Update EC members to point to the remaining relation instead of the removed
+ * one, removing duplicates.
+ *
+ * Restriction clauses for base relations are already distributed to
+ * the respective baserestrictinfo lists (see
+ * generate_implied_equalities_for_column). The above code has already processed
+ * this list, and updated these clauses to reference the remaining
+ * relation, so we can skip them here based on their relids.
+ *
+ * Likewise, we have already processed the join clauses that join the
+ * removed relation to the remaining one.
+ *
+ * Finally, there are join clauses that join the removed relation to
+ * some third relation. We can't just delete the source clauses and
+ * regenerate them from the EC because the corresponding equality
+ * operators might be missing (see the handling of ec_broken).
+ * Therefore, we will update the references in the source clauses.
+ *
+ * Derived clauses can be generated again, so it is simpler to just
+ * delete them.
+ */
+static void
+update_eclasses(EquivalenceClass *ec, int from, int to)
+{
+	List	   *new_members = NIL;
+	List	   *new_sources = NIL;
+	ListCell   *lc;
+	ListCell   *lc1;
+
+	foreach(lc, ec->ec_members)
+	{
+		EquivalenceMember *em = lfirst_node(EquivalenceMember, lc);
+		bool		is_redundant = false;
+
+		if (!bms_is_member(from, em->em_relids))
+		{
+			new_members = lappend(new_members, em);
+			continue;
+		}
+
+		em->em_relids = replace_relid(em->em_relids, from, to);
+		em->em_jdomain->jd_relids = replace_relid(em->em_jdomain->jd_relids, from, to);
+
+		/* We only process inner joins */
+		replace_varno((Node *) em->em_expr, from, to);
+
+		foreach(lc1, new_members)
+		{
+			EquivalenceMember *other = lfirst_node(EquivalenceMember, lc1);
+
+			if (!equal(em->em_relids, other->em_relids))
+				continue;
+
+			if (equal(em->em_expr, other->em_expr))
+			{
+				is_redundant = true;
+				break;
+			}
+		}
+
+		if (!is_redundant)
+			new_members = lappend(new_members, em);
+	}
+
+	list_free(ec->ec_members);
+	ec->ec_members = new_members;
+
+	list_free(ec->ec_derives);
+	ec->ec_derives = NULL;
+
+	/* Update EC source expressions */
+	foreach(lc, ec->ec_sources)
+	{
+		RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc);
+		bool		is_redundant = false;
+
+		if (!bms_is_member(from, rinfo->required_relids))
+		{
+			new_sources = lappend(new_sources, rinfo);
+			continue;
+		}
+
+		replace_varno((Node *) rinfo, from, to);
+
+		/*
+		 * After switching the clause to the remaining relation, check it for
+		 * redundancy with existing ones. We don't have to check for
+		 * redundancy with derived clauses, because we've just deleted them.
+		 */
+		foreach(lc1, new_sources)
+		{
+			RestrictInfo *other = lfirst_node(RestrictInfo, lc1);
+
+			if (!equal(rinfo->clause_relids, other->clause_relids))
+				continue;
+
+			if (equal(rinfo->clause, other->clause))
+			{
+				is_redundant = true;
+				break;
+			}
+		}
+
+		if (!is_redundant)
+			new_sources = lappend(new_sources, rinfo);
+	}
+
+	list_free(ec->ec_sources);
+	ec->ec_sources = new_sources;
+	ec->ec_relids = replace_relid(ec->ec_relids, from, to);
+}
+
+static bool
+sje_walker(Node *node, ReplaceVarnoContext *ctx)
+{
+	if (node == NULL)
+		return false;
+
+	if (IsA(node, Var))
+	{
+		Var		   *var = (Var *) node;
+
+		if (var->varno == ctx->from)
+		{
+			var->varno = ctx->to;
+			var->varnosyn = ctx->to;
+		}
+		return false;
+	}
+	return expression_tree_walker(node, sje_walker, (void *) ctx);
+}
+
+/*
+ * Remove a relation after we have proven that it participates only in an
+ * unneeded unique self join.
+ *
+ * Replace any links in planner info structures.
+ *
+ * Transfer join and restriction clauses from the removed relation to the
+ * remaining one. We change the Vars of the clause to point to the
+ * remaining relation instead of the removed one. The clauses that require
+ * a subset of joinrelids become restriction clauses of the remaining
+ * relation, and others remain join clauses. We append them to
+ * baserestrictinfo and joininfo respectively, trying not to introduce
+ * duplicates.
+ *
+ * We also have to process the 'joinclauses' list here, because it
+ * contains EC-derived join clauses which must become filter clauses. It
+ * is not enough to just correct the ECs because the EC-derived
+ * restrictions are generated before join removal (see
+ * generate_base_implied_equalities).
+ */
+static void
+remove_self_join_rel(PlannerInfo *root, PlanRowMark *kmark, PlanRowMark *rmark,
+					 RelOptInfo *toKeep, RelOptInfo *toRemove,
+					 List *restrictlist)
+{
+	List	   *joininfos;
+	ListCell   *lc;
+	int			i;
+	List	   *jinfo_candidates = NIL;
+	List	   *binfo_candidates = NIL;
+	ReplaceVarnoContext ctx = {.from = toRemove->relid,.to = toKeep->relid};
+
+	Assert(toKeep->relid != -1);
+
+	/*
+	 * Replace the index of the removing table with the keeping one. The
+	 * technique of removing/distributing restrictinfo is used here to attach
+	 * just appeared (for keeping relation) join clauses and avoid adding
+	 * duplicates of those that already exist in the joininfo list.
+	 */
+	joininfos = list_copy(toRemove->joininfo);
+	foreach(lc, joininfos)
+	{
+		RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc);
+
+		remove_join_clause_from_rels(root, rinfo, rinfo->required_relids);
+		replace_varno((Node *) rinfo, toRemove->relid, toKeep->relid);
+
+		if (bms_membership(rinfo->required_relids) == BMS_MULTIPLE)
+			jinfo_candidates = lappend(jinfo_candidates, rinfo);
+		else
+			binfo_candidates = lappend(binfo_candidates, rinfo);
+	}
+
+	/*
+	 * Concatenate restrictlist to the list of base restrictions of the
+	 * removing table just to simplify the replacement procedure: all of them
+	 * weren't connected to any keeping relations and need to be added to some
+	 * rels.
+	 */
+	toRemove->baserestrictinfo = list_concat(toRemove->baserestrictinfo,
+											 restrictlist);
+	foreach(lc, toRemove->baserestrictinfo)
+	{
+		RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc);
+
+		replace_varno((Node *) rinfo, toRemove->relid, toKeep->relid);
+
+		if (bms_membership(rinfo->required_relids) == BMS_MULTIPLE)
+			jinfo_candidates = lappend(jinfo_candidates, rinfo);
+		else
+			binfo_candidates = lappend(binfo_candidates, rinfo);
+	}
+
+	/*
+	 * Now, add all non-redundant clauses to the keeping relation.
+	 * Contradictory operation. On the one side, we reduce the length of
+	 * restrict lists that can impact planning or executing time.
+	 * Additionally, we improve the accuracy of cardinality estimation. On the
+	 * other side, it is one more place that can make planning time much
+	 * longer in specific cases. It would have been better to avoid calling
+	 * the equal() function here, but it's the only way to detect duplicated
+	 * inequality expressions.
+	 */
+	foreach(lc, binfo_candidates)
+	{
+		RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc);
+		ListCell   *olc = NULL;
+		bool		is_redundant = false;
+
+		Assert(!bms_is_member(toRemove->relid, rinfo->required_relids));
+
+		foreach(olc, toKeep->baserestrictinfo)
+		{
+			RestrictInfo *src = lfirst_node(RestrictInfo, olc);
+
+			if (!bms_equal(src->clause_relids, rinfo->clause_relids))
+				/* Const and non-const expressions can't be equal */
+				continue;
+
+			if (src == rinfo ||
+				(rinfo->parent_ec != NULL
+				 && src->parent_ec == rinfo->parent_ec)
+				|| equal(rinfo->clause, src->clause))
+			{
+				is_redundant = true;
+				break;
+			}
+		}
+		if (!is_redundant)
+			distribute_restrictinfo_to_rels(root, rinfo);
+	}
+	foreach(lc, jinfo_candidates)
+	{
+		RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc);
+		ListCell   *olc = NULL;
+		bool		is_redundant = false;
+
+		Assert(!bms_is_member(toRemove->relid, rinfo->required_relids));
+
+		foreach(olc, toKeep->joininfo)
+		{
+			RestrictInfo *src = lfirst_node(RestrictInfo, olc);
+
+			if (!bms_equal(src->clause_relids, rinfo->clause_relids))
+				/* Can't compare trivially different clauses */
+				continue;
+
+			if (src == rinfo ||
+				(rinfo->parent_ec != NULL
+				 && src->parent_ec == rinfo->parent_ec)
+				|| equal(rinfo->clause, src->clause))
+			{
+				is_redundant = true;
+				break;
+			}
+		}
+		if (!is_redundant)
+			distribute_restrictinfo_to_rels(root, rinfo);
+	}
+	list_free(binfo_candidates);
+	list_free(jinfo_candidates);
+
+	/*
+	 * Arrange equivalence classes, mentioned removing a table, with the
+	 * keeping one: varno of removing table should be replaced in members and
+	 * sources lists. Also, remove duplicated elements if this replacement
+	 * procedure created them.
+	 */
+	i = -1;
+	while ((i = bms_next_member(toRemove->eclass_indexes, i)) >= 0)
+	{
+		EquivalenceClass *ec = (EquivalenceClass *) list_nth(root->eq_classes, i);
+
+		update_eclasses(ec, toRemove->relid, toKeep->relid);
+		toKeep->eclass_indexes = bms_add_member(toKeep->eclass_indexes, i);
+	}
+
+	/*
+	 * Transfer the targetlist and attr_needed flags.
+	 */
+
+	foreach(lc, toRemove->reltarget->exprs)
+	{
+		Node	   *node = lfirst(lc);
+
+		replace_varno(node, toRemove->relid, toKeep->relid);
+		if (!list_member(toKeep->reltarget->exprs, node))
+			toKeep->reltarget->exprs = lappend(toKeep->reltarget->exprs, node);
+	}
+
+	for (i = toKeep->min_attr; i <= toKeep->max_attr; i++)
+	{
+		int			attno = i - toKeep->min_attr;
+
+		toRemove->attr_needed[attno] = replace_relid(toRemove->attr_needed[attno],
+													 toRemove->relid, toKeep->relid);
+		toKeep->attr_needed[attno] = bms_add_members(toKeep->attr_needed[attno],
+													 toRemove->attr_needed[attno]);
+	}
+
+	/*
+	 * If the removed relation has a row mark, transfer it to the remaining
+	 * one.
+	 *
+	 * If both rels have row marks, just keep the one corresponding to the
+	 * remaining relation, because we verified earlier that they have the same
+	 * strength.
+	 */
+	if (rmark)
+	{
+		if (kmark)
+		{
+			Assert(kmark->markType == rmark->markType);
+
+			root->rowMarks = list_delete_ptr(root->rowMarks, rmark);
+		}
+		else
+		{
+			/* Shouldn't have inheritance children here. */
+			Assert(rmark->rti == rmark->prti);
+
+			rmark->rti = rmark->prti = toKeep->relid;
+		}
+	}
+
+	/* Replace varno in all the query structures */
+	query_tree_walker(root->parse, sje_walker, &ctx, QTW_EXAMINE_SORTGROUP);
+
+	/* Replace links in the planner info */
+	remove_rel_from_query(root, toRemove, toKeep->relid, NULL, NULL);
+
+	/* At last, replace varno in root targetlist and HAVING clause */
+	replace_varno((Node *) root->processed_tlist,
+				  toRemove->relid, toKeep->relid);
+	replace_varno((Node *) root->processed_groupClause,
+				  toRemove->relid, toKeep->relid);
+
+	/*
+	 * There may be references to the rel in root->fkey_list, but if so,
+	 * match_foreign_keys_to_quals() will get rid of them.
+	 */
+
+	/*
+	 * Finally, remove the rel from the baserel array to prevent it from being
+	 * referenced again.  (We can't do this earlier because
+	 * remove_join_clause_from_rels will touch it.)
+	 */
+	root->simple_rel_array[toRemove->relid] = NULL;
+
+	/* And nuke the RelOptInfo, just in case there's another access path */
+	pfree(toRemove);
+}
+
+/*
+ * split_selfjoin_quals
+ *		Processes 'joinquals' by building two lists: one containing the quals
+ *		where the columns/exprs are on either side of the join match, and
+ *		another one containing the remaining quals.
+ *
+ * 'joinquals' must only contain quals for a RTE_RELATION being joined to
+ * itself.
+ */
+static void
+split_selfjoin_quals(PlannerInfo *root, List *joinquals, List **selfjoinquals,
+					 List **otherjoinquals, int from, int to)
+{
+	ListCell   *lc;
+	List	   *sjoinquals = NIL;
+	List	   *ojoinquals = NIL;
+
+	foreach(lc, joinquals)
+	{
+		RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc);
+		OpExpr	   *expr;
+		Node	   *leftexpr;
+		Node	   *rightexpr;
+
+		/* In general, clause looks like F(arg1) = G(arg2) */
+		if (!rinfo->mergeopfamilies ||
+			bms_num_members(rinfo->clause_relids) != 2 ||
+			bms_membership(rinfo->left_relids) != BMS_SINGLETON ||
+			bms_membership(rinfo->right_relids) != BMS_SINGLETON)
+		{
+			ojoinquals = lappend(ojoinquals, rinfo);
+			continue;
+		}
+
+		expr = (OpExpr *) rinfo->clause;
+
+		if (!IsA(expr, OpExpr) || list_length(expr->args) != 2)
+		{
+			ojoinquals = lappend(ojoinquals, rinfo);
+			continue;
+		}
+
+		leftexpr = get_leftop(rinfo->clause);
+		rightexpr = copyObject(get_rightop(rinfo->clause));
+
+		if (leftexpr && IsA(leftexpr, RelabelType))
+			leftexpr = (Node *) ((RelabelType *) leftexpr)->arg;
+		if (rightexpr && IsA(rightexpr, RelabelType))
+			rightexpr = (Node *) ((RelabelType *) rightexpr)->arg;
+
+		/*
+		 * Quite an expensive operation, narrowing the use case. For example,
+		 * when we have cast of the same var to different (but compatible)
+		 * types.
+		 */
+		replace_varno(rightexpr, bms_singleton_member(rinfo->right_relids),
+					  bms_singleton_member(rinfo->left_relids));
+
+		if (equal(leftexpr, rightexpr))
+			sjoinquals = lappend(sjoinquals, rinfo);
+		else
+			ojoinquals = lappend(ojoinquals, rinfo);
+	}
+
+	*selfjoinquals = sjoinquals;
+	*otherjoinquals = ojoinquals;
+}
+
+/*
+ * Check for a case when uniqueness is at least partly derived from a
+ * baserestrictinfo clause. In this case, we have a chance to return only
+ * one row (if such clauses on both sides of SJ are equal) or nothing (if they
+ * are different).
+ */
+static bool
+match_unique_clauses(PlannerInfo *root, RelOptInfo *outer, List *uclauses,
+					 Index relid)
+{
+	ListCell   *lc;
+
+	foreach(lc, uclauses)
+	{
+		RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc);
+		Expr	   *clause;
+		Node	   *iclause;
+		Node	   *c1;
+		bool		matched = false;
+		ListCell   *olc;
+
+		Assert(outer->relid > 0 && relid > 0);
+
+		/* Only filters like f(R.x1,...,R.xN) == expr we should consider. */
+		Assert(bms_is_empty(rinfo->left_relids) ^
+			   bms_is_empty(rinfo->right_relids));
+
+		clause = (Expr *) copyObject(rinfo->clause);
+		replace_varno((Node *) clause, relid, outer->relid);
+
+		iclause = bms_is_empty(rinfo->left_relids) ? get_rightop(clause) :
+			get_leftop(clause);
+		c1 = bms_is_empty(rinfo->left_relids) ? get_leftop(clause) :
+			get_rightop(clause);
+
+		/*
+		 * Compare these left and right sides with the corresponding sides of
+		 * the outer's filters. If no one is detected - return immediately.
+		 */
+		foreach(olc, outer->baserestrictinfo)
+		{
+			RestrictInfo *orinfo = lfirst_node(RestrictInfo, olc);
+			Node	   *oclause;
+			Node	   *c2;
+
+			if (orinfo->mergeopfamilies == NIL)
+				/* Don't consider clauses which aren't similar to 'F(X)=G(Y)' */
+				continue;
+
+			Assert(is_opclause(orinfo->clause));
+
+			oclause = bms_is_empty(orinfo->left_relids) ?
+				get_rightop(orinfo->clause) : get_leftop(orinfo->clause);
+			c2 = (bms_is_empty(orinfo->left_relids) ?
+				  get_leftop(orinfo->clause) : get_rightop(orinfo->clause));
+
+			if (equal(iclause, oclause) && equal(c1, c2))
+			{
+				matched = true;
+				break;
+			}
+		}
+
+		if (!matched)
+			return false;
+	}
+
+	return true;
+}
+
+/*
+ * Find and remove unique self joins in a group of base relations that have
+ * the same Oid.
+ *
+ * Returns a set of relids that were removed.
+ */
+static Relids
+remove_self_joins_one_group(PlannerInfo *root, Relids relids)
+{
+	Relids		result = NULL;
+	int			k;				/* Index of kept relation */
+	int			r = -1;			/* Index of removed relation */
+
+	while ((r = bms_next_member(relids, r)) > 0)
+	{
+		RelOptInfo *inner = root->simple_rel_array[r];
+
+		k = r;
+
+		while ((k = bms_next_member(relids, k)) > 0)
+		{
+			Relids		joinrelids = NULL;
+			RelOptInfo *outer = root->simple_rel_array[k];
+			List	   *restrictlist;
+			List	   *selfjoinquals;
+			List	   *otherjoinquals;
+			ListCell   *lc;
+			bool		jinfo_check = true;
+			PlanRowMark *omark = NULL;
+			PlanRowMark *imark = NULL;
+			List	   *uclauses = NIL;
+
+			/* A sanity check: the relations have the same Oid. */
+			Assert(root->simple_rte_array[k]->relid ==
+				   root->simple_rte_array[r]->relid);
+
+			/*
+			 * It is impossible to eliminate join of two relations if they
+			 * belong to different rules of order. Otherwise planner can't be
+			 * able to find any variants of correct query plan.
+			 */
+			foreach(lc, root->join_info_list)
+			{
+				SpecialJoinInfo *info = (SpecialJoinInfo *) lfirst(lc);
+
+				if ((bms_is_member(k, info->syn_lefthand) ^
+					 bms_is_member(r, info->syn_lefthand)) ||
+					(bms_is_member(k, info->syn_righthand) ^
+					 bms_is_member(r, info->syn_righthand)))
+				{
+					jinfo_check = false;
+					break;
+				}
+			}
+			if (!jinfo_check)
+				continue;
+
+			/*
+			 * Check Row Marks equivalence.  We can't remove the join if the
+			 * relations have row marks of different strength (e.g. one is
+			 * locked FOR UPDATE and another just has ROW_MARK_REFERENCE for
+			 * EvalPlanQual rechecking).
+			 */
+			foreach(lc, root->rowMarks)
+			{
+				PlanRowMark *rowMark = (PlanRowMark *) lfirst(lc);
+
+				if (rowMark->rti == k)
+				{
+					Assert(imark == NULL);
+					imark = rowMark;
+				}
+				else if (rowMark->rti == r)
+				{
+					Assert(omark == NULL);
+					omark = rowMark;
+				}
+
+				if (omark && imark)
+					break;
+			}
+			if (omark && imark && omark->markType != imark->markType)
+				continue;
+
+			/*
+			 * We only deal with base rels here, so their relids bitset
+			 * contains only one member -- their relid.
+			 */
+			joinrelids = bms_add_member(joinrelids, r);
+			joinrelids = bms_add_member(joinrelids, k);
+
+			/*
+			 * Be safe to do not remove tables participated in complicated PH
+			 */
+			foreach(lc, root->placeholder_list)
+			{
+				PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);
+
+				/* there isn't any other place to eval PHV */
+				if (bms_is_subset(phinfo->ph_eval_at, joinrelids) ||
+					bms_is_subset(phinfo->ph_needed, joinrelids))
+					break;
+			}
+			if (lc)
+				continue;
+
+			/*
+			 * At this stage, joininfo lists of inner and outer can contain
+			 * only clauses, required for a superior outer join that can't
+			 * influence this optimization. So, we can avoid to call the
+			 * build_joinrel_restrictlist() routine.
+			 */
+			restrictlist = generate_join_implied_equalities(root, joinrelids,
+															inner->relids,
+															outer, NULL);
+
+			/*
+			 * Process restrictlist to separate the self join quals out of the
+			 * other quals. e.g x = x goes to selfjoinquals and a = b to
+			 * otherjoinquals.
+			 */
+			split_selfjoin_quals(root, restrictlist, &selfjoinquals,
+								 &otherjoinquals, inner->relid, outer->relid);
+
+			/*
+			 * To enable SJE for the only degenerate case without any self
+			 * join clauses at all, add baserestrictinfo to this list. The
+			 * degenerate case works only if both sides have the same clause.
+			 * So doesn't matter which side to add.
+			 */
+			selfjoinquals = list_concat(selfjoinquals, outer->baserestrictinfo);
+
+			/*
+			 * Determine if the inner table can duplicate outer rows.  We must
+			 * bypass the unique rel cache here since we're possibly using a
+			 * subset of join quals. We can use 'force_cache' == true when all
+			 * join quals are self-join quals.  Otherwise, we could end up
+			 * putting false negatives in the cache.
+			 */
+			if (!innerrel_is_unique_ext(root, joinrelids, inner->relids,
+										outer, JOIN_INNER, selfjoinquals,
+										list_length(otherjoinquals) == 0,
+										&uclauses))
+				continue;
+
+			/*
+			 * We have proven that for both relations, the same unique index
+			 * guarantees that there is at most one row where columns equal
+			 * given values. These values must be the same for both relations,
+			 * or else we won't match the same row on each side of the join.
+			 */
+			if (!match_unique_clauses(root, inner, uclauses, outer->relid))
+				continue;
+
+			/*
+			 * We can remove either relation, so remove the inner one in order
+			 * to simplify this loop.
+			 */
+			remove_self_join_rel(root, omark, imark, outer, inner, restrictlist);
+
+			result = bms_add_member(result, r);
+
+			/* We have removed the outer relation, try the next one. */
+			break;
+		}
+	}
+
+	return result;
+}
+
+/*
+ * Gather indexes of base relations from the joinlist and try to eliminate self
+ * joins. To avoid complexity, limit the max power of this set by a GUC.
+ */
+static Relids
+remove_self_joins_recurse(PlannerInfo *root, List *joinlist, Relids toRemove)
+{
+	ListCell   *jl;
+	Relids		relids = NULL;
+	SelfJoinCandidate *candidates = NULL;
+	int			i;
+	int			j;
+	int			numRels;
+
+	/* Collect indexes of base relations of the join tree */
+	foreach(jl, joinlist)
+	{
+		Node	   *jlnode = (Node *) lfirst(jl);
+
+		if (IsA(jlnode, RangeTblRef))
+		{
+			RangeTblRef *ref = (RangeTblRef *) jlnode;
+			RangeTblEntry *rte = root->simple_rte_array[ref->rtindex];
+
+			/*
+			 * We only care about base relations from which we select
+			 * something.
+			 */
+			if (rte->rtekind == RTE_RELATION &&
+				rte->relkind == RELKIND_RELATION &&
+				root->simple_rel_array[ref->rtindex] != NULL)
+			{
+				Assert(!bms_is_member(ref->rtindex, relids));
+				relids = bms_add_member(relids, ref->rtindex);
+			}
+		}
+		else if (IsA(jlnode, List))
+			/* Recursively go inside the sub-joinlist */
+			toRemove = remove_self_joins_recurse(root, (List *) jlnode,
+												 toRemove);
+		else
+			elog(ERROR, "unrecognized joinlist node type: %d",
+				 (int) nodeTag(jlnode));
+	}
+
+	numRels = bms_num_members(relids);
+
+	/* Need at least two relations for the join */
+	if (numRels < 2)
+		return toRemove;
+
+	/*
+	 * In order to find relations with the same oid we first build an array of
+	 * candidates and then sort it by oid.
+	 */
+	candidates = (SelfJoinCandidate *) palloc(sizeof(SelfJoinCandidate) *
+											  numRels);
+	i = -1;
+	j = 0;
+	while ((i = bms_next_member(relids, i)) >= 0)
+	{
+		candidates[j].relid = i;
+		candidates[j].reloid = root->simple_rte_array[i]->relid;
+		j++;
+	}
+
+	pg_qsort(candidates, numRels, sizeof(SelfJoinCandidate),
+			 self_join_candidates_cmp);
+
+	/*
+	 * Iteratively form a group of relation indexes with the same oid and
+	 * launch the routine that detects self-joins in this group and removes
+	 * excessive range table entries.
+	 *
+	 * At the end of the iteration, exclude the group from the overall relids
+	 * list. So each next iteration of the cycle will involve less and less
+	 * value of relids.
+	 */
+	i = 0;
+	for (j = 1; j < numRels + 1; j++)
+	{
+		if (j == numRels || candidates[j].reloid != candidates[i].reloid)
+		{
+			if (j - i >= 2)
+			{
+				/* Create a group of relation indexes with the same oid */
+				Relids		group = NULL;
+				Relids		removed;
+
+				while (i < j)
+				{
+					group = bms_add_member(group, candidates[i].relid);
+					i++;
+				}
+
+				relids = bms_del_members(relids, group);
+
+				/*
+				 * Try to remove self-joins from a group of identical entries.
+				 * Make the next attempt iteratively - if something is deleted
+				 * from a group, changes in clauses and equivalence classes
+				 * can give us a chance to find more candidates.
+				 */
+				do
+				{
+					Assert(!bms_overlap(group, toRemove));
+					removed = remove_self_joins_one_group(root, group);
+					toRemove = bms_add_members(toRemove, removed);
+					group = bms_del_members(group, removed);
+				} while (!bms_is_empty(removed) &&
+						 bms_membership(group) == BMS_MULTIPLE);
+				bms_free(removed);
+				bms_free(group);
+			}
+			else
+			{
+				/* Single relation, just remove it from the set */
+				relids = bms_del_member(relids, candidates[i].relid);
+				i = j;
+			}
+		}
+	}
+
+	Assert(bms_is_empty(relids));
+
+	return toRemove;
+}
+
+/*
+ * Compare self-join candidates by their oids.
+ */
+static int
+self_join_candidates_cmp(const void *a, const void *b)
+{
+	const SelfJoinCandidate *ca = (const SelfJoinCandidate *) a;
+	const SelfJoinCandidate *cb = (const SelfJoinCandidate *) b;
+
+	if (ca->reloid != cb->reloid)
+		return (ca->reloid < cb->reloid ? -1 : 1);
+	else
+		return 0;
+}
+
+/*
+ * Find and remove useless self joins.
+ *
+ * Search for joins where a relation is joined to itself. If the join clause
+ * for each tuple from one side of the join is proven to match the same
+ * physical row (or nothing) on the other side, that self-join can be
+ * eliminated from the query.  Suitable join clauses are assumed to be in the
+ * form of X = X, and can be replaced with NOT NULL clauses.
+ *
+ * For the sake of simplicity, we don't apply this optimization to special
+ * joins. Here is a list of what we could do in some particular cases:
+ * 'a a1 semi join a a2': is reduced to inner by reduce_unique_semijoins,
+ * and then removed normally.
+ * 'a a1 anti join a a2': could simplify to a scan with 'outer quals AND
+ * (IS NULL on join columns OR NOT inner quals)'.
+ * 'a a1 left join a a2': could simplify to a scan like inner but without
+ * NOT NULL conditions on join columns.
+ * 'a a1 left join (a a2 join b)': can't simplify this, because join to b
+ * can both remove rows and introduce duplicates.
+ *
+ * To search for removable joins, we order all the relations on their Oid,
+ * go over each set with the same Oid, and consider each pair of relations
+ * in this set.
+ *
+ * To remove the join, we mark one of the participating relations as dead
+ * and rewrite all references to it to point to the remaining relation.
+ * This includes modifying RestrictInfos, EquivalenceClasses, and
+ * EquivalenceMembers. We also have to modify the row marks. The join clauses
+ * of the removed relation become either restriction or join clauses, based on
+ * whether they reference any relations not participating in the removed join.
+ *
+ * 'targetlist' is the top-level targetlist of the query. If it has any
+ * references to the removed relations, we update them to point to the
+ * remaining ones.
+ */
+List *
+remove_useless_self_joins(PlannerInfo *root, List *joinlist)
+{
+	Relids		toRemove = NULL;
+	int			relid = -1;
+
+	if (!enable_self_join_removal || joinlist == NIL ||
+		(list_length(joinlist) == 1 && !IsA(linitial(joinlist), List)))
+		return joinlist;
+
+	/*
+	 * Merge pairs of relations participated in self-join. Remove unnecessary
+	 * range table entries.
+	 */
+	toRemove = remove_self_joins_recurse(root, joinlist, toRemove);
+
+	if (unlikely(toRemove != NULL))
+	{
+		int			nremoved = 0;
+
+		/* At the end, remove orphaned relation links */
+		while ((relid = bms_next_member(toRemove, relid)) >= 0)
+			joinlist = remove_rel_from_joinlist(joinlist, relid, &nremoved);
+	}
+
+	return joinlist;
 }