From 8cd282f6c0d1f807a2b454ebea0b1482ecb523a2 Mon Sep 17 00:00:00 2001
From: Tomas Vondra <tomas@vondra.me>
Date: Fri, 17 Jul 2026 23:06:05 +0200
Subject: [PATCH v10 09/21] Allow filters built for queries with LATERAL joins

Until now enumerate_bloom_filter_build_relids simply disabled join
enumeration for queries with LATERAL joins, to keep the enumeration
simple. It still considered filters on base relations, at least.

This extends the enumeration to handle LATERAL joins correctly, by
cloning the logic from join_is_legal and adjusting it for relids.
---
 src/backend/optimizer/path/joinrels.c | 211 ++++++++++++++++++++++++--
 src/test/regress/expected/join.out    |  34 ++---
 2 files changed, 215 insertions(+), 30 deletions(-)

diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index 8faba70664b..e0ede240822 100644
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -649,6 +649,88 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 	return true;
 }
 
+/*
+ * simple_lateral_relids
+ *	  Compute the lateral_relids of an arbitrary set of base relations, that
+ *	  is, the set of rels *outside* "relids" that members of "relids" have
+ *	  LATERAL references to.
+ *
+ * This mirrors how build_join_rel()/min_join_parameterization() derive a
+ * joinrel's lateral_relids from its inputs: take the union of the member base
+ * relations' (already transitively-closed) lateral_relids and remove the
+ * members themselves.  It lets join_is_legal_relids() reason about LATERAL
+ * without having a RelOptInfo for a multi-rel set.
+ */
+static Relids
+simple_lateral_relids(PlannerInfo *root, Relids relids)
+{
+	Relids		result = NULL;
+	int			i;
+
+	i = -1;
+	while ((i = bms_next_member(relids, i)) >= 0)
+	{
+		RelOptInfo *rel = root->simple_rel_array[i];
+
+		/*
+		 * Relids should contain base relations only, and those should all
+		 * have a valid RelOptInfo already.
+		 */
+		Assert(rel != NULL);
+
+		result = bms_add_members(result, rel->lateral_relids);
+	}
+
+	return bms_del_members(result, relids);
+}
+
+/*
+ * simple_direct_lateral_relids
+ *	  Like relids_lateral_relids(), but for direct_lateral_relids (references
+ *	  that are direct, i.e. not through an intermediate rel).  See
+ *	  build_join_rel(), which likewise unions the inputs' direct_lateral_relids
+ *	  and removes the join's own relids.
+ */
+static Relids
+simple_direct_lateral_relids(PlannerInfo *root, Relids relids)
+{
+	Relids		result = NULL;
+	int			i;
+
+	i = -1;
+	while ((i = bms_next_member(relids, i)) >= 0)
+	{
+		RelOptInfo *rel = root->simple_rel_array[i];
+
+		/*
+		 * Relids should contain base relations only, and those should all
+		 * have a valid RelOptInfo already.
+		 */
+		Assert(rel != NULL);
+
+		result = bms_add_members(result, rel->direct_lateral_relids);
+	}
+
+	return bms_del_members(result, relids);
+}
+
+/*
+ * simple_min_join_parameterization
+ *		Like min_join_parameterization(), but operates on Relids directly,
+ *		without having RelOptInfo.
+ */
+static Relids
+simple_min_join_parameterization(PlannerInfo *root, Relids joinrelids,
+								 Relids lateral_relids1, Relids lateral_relids2)
+{
+	Relids		result = NULL;
+
+	result = bms_union(lateral_relids1, lateral_relids2);
+	result = bms_del_members(result, joinrelids);
+
+	return result;
+}
+
 /*
  * simple_join_is_legal
  *	  Relids-only variant of join_is_legal(), used by the Bloom-filter
@@ -656,18 +738,18 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
  *	  legally be joined into a single join relation, without building any
  *	  RelOptInfo or generating paths.
  *
- * This mirrors the special-join legality logic of join_is_legal(), operating
- * purely on relid sets.  Two simplifications are made relative to the real
- * function:
+ * This mirrors the special-join and LATERAL legality logic of join_is_legal(),
+ * operating purely on relid sets.  One simplification is made relative to the
+ * real function:
  *
  *	- Semijoin unique-ification, which join_is_legal() probes via
  *	  create_unique_paths(), is approximated by the sjinfo's semi_can_btree /
  *	  semi_can_hash flags (create_unique_paths() ultimately succeeds only when
  *	  one of those holds; see planner.c).
  *
- *	- LATERAL handling is omitted; callers must not use this on queries with
- *	  lateral references (see enumerate_bloom_filter_build_relids, which skips
- *	  the whole feature when hasLateralRTEs is set).
+ * LATERAL constraints are handled the same way as in join_is_legal(), using
+ * lateral_relids / direct_lateral_relids computed for the (possibly multi-rel)
+ * input sets by relids_lateral_relids() / relids_direct_lateral_relids().
  *
  * The result is purely advisory: it is only used to decide which build sides
  * to consider for Bloom filters.  Any filter that turns out not to be
@@ -680,12 +762,14 @@ simple_join_is_legal(PlannerInfo *root, Relids relids1, Relids relids2,
 					 Relids joinrelids)
 {
 	SpecialJoinInfo *match_sjinfo;
+	bool		reversed;
+	bool		unique_ified;
 	bool		must_be_leftjoin;
 	ListCell   *l;
 
-	Assert(!root->hasLateralRTEs);
-
 	match_sjinfo = NULL;
+	reversed = false;
+	unique_ified = false;
 	must_be_leftjoin = false;
 
 	foreach(l, root->join_info_list)
@@ -746,7 +830,7 @@ simple_join_is_legal(PlannerInfo *root, Relids relids1, Relids relids2,
 			if (match_sjinfo)
 				return false;	/* invalid join path */
 			match_sjinfo = sjinfo;
-			/* reversed = false; */
+			reversed = false;
 		}
 		else if (bms_is_subset(sjinfo->min_lefthand, relids2) &&
 				 bms_is_subset(sjinfo->min_righthand, relids1))
@@ -754,7 +838,7 @@ simple_join_is_legal(PlannerInfo *root, Relids relids1, Relids relids2,
 			if (match_sjinfo)
 				return false;	/* invalid join path */
 			match_sjinfo = sjinfo;
-			/* reversed = true; */
+			reversed = true;
 		}
 		else if (sjinfo->jointype == JOIN_SEMI &&
 				 bms_equal(sjinfo->syn_righthand, relids2) &&
@@ -787,6 +871,8 @@ simple_join_is_legal(PlannerInfo *root, Relids relids1, Relids relids2,
 			if (match_sjinfo)
 				return false;	/* invalid join path */
 			match_sjinfo = sjinfo;
+			reversed = false;
+			unique_ified = true;
 		}
 		else if (sjinfo->jointype == JOIN_SEMI &&
 				 bms_equal(sjinfo->syn_righthand, relids1) &&
@@ -797,6 +883,8 @@ simple_join_is_legal(PlannerInfo *root, Relids relids1, Relids relids2,
 			if (match_sjinfo)
 				return false;	/* invalid join path */
 			match_sjinfo = sjinfo;
+			reversed = true;
+			unique_ified = true;
 		}
 		else
 		{
@@ -851,6 +939,108 @@ simple_join_is_legal(PlannerInfo *root, Relids relids1, Relids relids2,
 		 !match_sjinfo->lhs_strict))
 		return false;			/* invalid join path */
 
+	/*
+	 * We also have to check for constraints imposed by LATERAL references.
+	 * This mirrors the corresponding logic in join_is_legal(), except that we
+	 * derive the input sets' lateral_relids / direct_lateral_relids from
+	 * their member base relations rather than from a prebuilt RelOptInfo.
+	 */
+	if (root->hasLateralRTEs)
+	{
+		Relids		lateral_relids1 = simple_lateral_relids(root, relids1);
+		Relids		lateral_relids2 = simple_lateral_relids(root, relids2);
+		bool		lateral_fwd;
+		bool		lateral_rev;
+		Relids		join_lateral_rels;
+
+		/*
+		 * The proposed rels could each contain lateral references to the
+		 * other, in which case the join is impossible.  If there are lateral
+		 * references in just one direction, then the join has to be done with
+		 * a nestloop with the lateral referencer on the inside.  If the join
+		 * matches an SJ that cannot be implemented by such a nestloop, the
+		 * join is impossible.
+		 *
+		 * Also, if the lateral reference is only indirect, we should reject
+		 * the join; whatever rel(s) the reference chain goes through must be
+		 * joined to first.
+		 */
+		lateral_fwd = bms_overlap(relids1, lateral_relids2);
+		lateral_rev = bms_overlap(relids2, lateral_relids1);
+		if (lateral_fwd && lateral_rev)
+			return false;		/* have lateral refs in both directions */
+		if (lateral_fwd)
+		{
+			/* has to be implemented as nestloop with relids1 on left */
+			if (match_sjinfo &&
+				(reversed ||
+				 unique_ified ||
+				 match_sjinfo->jointype == JOIN_FULL))
+				return false;	/* not implementable as nestloop */
+			/* check there is a direct reference from relids2 to relids1 */
+			if (!bms_overlap(relids1,
+							 simple_direct_lateral_relids(root, relids2)))
+				return false;	/* only indirect refs, so reject */
+		}
+		else if (lateral_rev)
+		{
+			/* has to be implemented as nestloop with relids2 on left */
+			if (match_sjinfo &&
+				(!reversed ||
+				 unique_ified ||
+				 match_sjinfo->jointype == JOIN_FULL))
+				return false;	/* not implementable as nestloop */
+			/* check there is a direct reference from relids1 to relids2 */
+			if (!bms_overlap(relids2,
+							 simple_direct_lateral_relids(root, relids1)))
+				return false;	/* only indirect refs, so reject */
+		}
+
+		/*
+		 * LATERAL references could also cause problems later on if we accept
+		 * this join: if the join's minimum parameterization includes any rels
+		 * that would have to be on the inside of an outer join with this join
+		 * rel, then it's never going to be possible to build the complete
+		 * query using this join.  We should reject this join not only because
+		 * it'll save work, but because if we don't, the clauseless-join
+		 * heuristics might think that legality of this join means that some
+		 * other join rel need not be formed, and that could lead to failure
+		 * to find any plan at all.  We have to consider not only rels that
+		 * are directly on the inner side of an OJ with the joinrel, but also
+		 * ones that are indirectly so, so search to find all such rels.
+		 */
+		join_lateral_rels = simple_min_join_parameterization(root, joinrelids,
+															 lateral_relids1, lateral_relids2);
+		if (join_lateral_rels)
+		{
+			Relids		join_plus_rhs = bms_copy(joinrelids);
+			bool		more;
+
+			do
+			{
+				more = false;
+				foreach(l, root->join_info_list)
+				{
+					SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(l);
+
+					/* ignore full joins --- their ordering is predetermined */
+					if (sjinfo->jointype == JOIN_FULL)
+						continue;
+
+					if (bms_overlap(sjinfo->min_lefthand, join_plus_rhs) &&
+						!bms_is_subset(sjinfo->min_righthand, join_plus_rhs))
+					{
+						join_plus_rhs = bms_add_members(join_plus_rhs,
+														sjinfo->min_righthand);
+						more = true;
+					}
+				}
+			} while (more);
+			if (bms_overlap(join_plus_rhs, join_lateral_rels))
+				return false;	/* will not be able to join to some RHS rel */
+		}
+	}
+
 	/* Otherwise, it's a valid join */
 	return true;
 }
@@ -1003,7 +1193,6 @@ enumerate_bloom_filter_build_relids(PlannerInfo *root)
 	 * fall back to GEQO, whose plans this speculative work would not help.
 	 */
 	if (nbaserels < 2 ||
-		root->hasLateralRTEs ||
 		(enable_geqo && nbaserels >= geqo_threshold))
 	{
 		root->bloom_build_relids = levels[1];
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 32fbad6f5ba..926d68d8b5b 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -4777,11 +4777,10 @@ select ss1.d1 from
     on i8.q1 = i4.f1
   on t1.tenthous = ss1.d1
 where t1.unique1 < i4.f1;
-                                                                                                             QUERY PLAN                                                                                                              
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+                                                                                                          QUERY PLAN                                                                                                           
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Nested Loop
    Output: (64)::information_schema.cardinal_number
-   Join Filter: (t1.tenthous = ((64)::information_schema.cardinal_number)::integer)
    ->  Seq Scan on public.tenk1 t3
          Output: t3.unique1, t3.unique2, t3.two, t3.four, t3.ten, t3.twenty, t3.hundred, t3.thousand, t3.twothousand, t3.fivethous, t3.tenthous, t3.odd, t3.even, t3.stringu1, t3.stringu2, t3.string4
          Filter: (t3.fivethous < 0)
@@ -4790,29 +4789,26 @@ where t1.unique1 < i4.f1;
          ->  Nested Loop
                Output: t1.tenthous, t2.ten, i4.f1
                Join Filter: (t1.unique1 < i4.f1)
-               ->  Hash Join
+               ->  Nested Loop
                      Output: t1.tenthous, t1.unique1, t2.ten
-                     Hash Cond: (t2.ten = t1.tenthous)
-                     ->  Seq Scan on public.tenk1 t2
-                           Output: t2.unique1, t2.unique2, t2.two, t2.four, t2.ten, t2.twenty, t2.hundred, t2.thousand, t2.twothousand, t2.fivethous, t2.tenthous, t2.odd, t2.even, t2.stringu1, t2.stringu2, t2.string4
-                     ->  Hash
-                           Output: t1.tenthous, t1.unique1
-                           ->  Nested Loop
-                                 Output: t1.tenthous, t1.unique1
-                                 ->  Subquery Scan on ss0
-                                       Output: ss0.x, (64)::information_schema.cardinal_number
-                                       ->  Result
-                                             Output: ((abs(t3.unique1))::double precision + random())
-                                 ->  Index Scan using tenk1_thous_tenthous on public.tenk1 t1
-                                       Output: t1.unique1, t1.unique2, t1.two, t1.four, t1.ten, t1.twenty, t1.hundred, t1.thousand, t1.twothousand, t1.fivethous, t1.tenthous, t1.odd, t1.even, t1.stringu1, t1.stringu2, t1.string4
-                                       Index Cond: (t1.tenthous = (((64)::information_schema.cardinal_number))::integer)
+                     Join Filter: (t1.tenthous = ((64)::information_schema.cardinal_number)::integer)
+                     ->  Nested Loop
+                           Output: t2.ten
+                           Join Filter: (((64)::information_schema.cardinal_number)::integer = t2.ten)
+                           ->  Result
+                                 Output: ((abs(t3.unique1))::double precision + random())
+                           ->  Seq Scan on public.tenk1 t2
+                                 Output: t2.unique1, t2.unique2, t2.two, t2.four, t2.ten, t2.twenty, t2.hundred, t2.thousand, t2.twothousand, t2.fivethous, t2.tenthous, t2.odd, t2.even, t2.stringu1, t2.stringu2, t2.string4
+                     ->  Index Scan using tenk1_thous_tenthous on public.tenk1 t1
+                           Output: t1.unique1, t1.unique2, t1.two, t1.four, t1.ten, t1.twenty, t1.hundred, t1.thousand, t1.twothousand, t1.fivethous, t1.tenthous, t1.odd, t1.even, t1.stringu1, t1.stringu2, t1.string4
+                           Index Cond: (t1.tenthous = t2.ten)
                ->  Seq Scan on public.int4_tbl i4
                      Output: i4.f1
                      Filter: (i4.f1 = ((64)::information_schema.cardinal_number)::integer)
          ->  Seq Scan on public.int8_tbl i8
                Output: i8.q1, i8.q2
                Filter: (i8.q1 = ((64)::information_schema.cardinal_number)::integer)
-(33 rows)
+(29 rows)
 
 select ss1.d1 from
   tenk1 as t1
-- 
2.50.1 (Apple Git-155)

