0001-Re-enable-parallel-plans-for-RIGHT_SEMI-joins.patch

From 3616f8f9e6141c36116f9c0068ef2aa44182109c Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Tue, 11 Nov 2025 13:43:58 +1300
Subject: [PATCH 1/2] Re-enable parallel plans for RIGHT_SEMI joins.

This reverts commit 257ee78341f2657d5c19cdaf0888f843e9bb0c33.
---
 src/backend/optimizer/path/joinpath.c | 21 ++++++---------------
 src/test/regress/expected/join.out    | 27 ---------------------------
 src/test/regress/sql/join.sql         | 20 --------------------
 3 files changed, 6 insertions(+), 62 deletions(-)

diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index ea5b6415186..3b9407eb2eb 100644
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -2260,20 +2260,10 @@ hash_inner_and_outer(PlannerInfo *root,
 
 		/*
 		 * If the joinrel is parallel-safe, we may be able to consider a
-		 * partial hash join.
-		 *
-		 * However, we can't handle JOIN_RIGHT_SEMI, because the hash table is
-		 * either a shared hash table or a private hash table per backend.  In
-		 * the shared case, there is no concurrency protection for the match
-		 * flags, so multiple workers could inspect and set the flags
-		 * concurrently, potentially producing incorrect results.  In the
-		 * private case, each worker has its own copy of the hash table, so no
-		 * single process has all the match flags.
-		 *
-		 * Also, the resulting path must not be parameterized.
+		 * partial hash join.  However, the resulting path must not be
+		 * parameterized.
 		 */
 		if (joinrel->consider_parallel &&
-			jointype != JOIN_RIGHT_SEMI &&
 			outerrel->partial_pathlist != NIL &&
 			bms_is_empty(joinrel->lateral_relids))
 		{
@@ -2304,12 +2294,13 @@ hash_inner_and_outer(PlannerInfo *root,
 			 * Normally, given that the joinrel is parallel-safe, the cheapest
 			 * total inner path will also be parallel-safe, but if not, we'll
 			 * have to search for the cheapest safe, unparameterized inner
-			 * path.  If full, right, or right-anti join, we can't use
-			 * parallelism (building the hash table in each backend) because
-			 * no one process has all the match bits.
+			 * path.  If full, right, right-semi or right-anti join, we can't
+			 * use parallelism (building the hash table in each backend)
+			 * because no one process has all the match bits.
 			 */
 			if (jointype == JOIN_FULL ||
 				jointype == JOIN_RIGHT ||
+				jointype == JOIN_RIGHT_SEMI ||
 				jointype == JOIN_RIGHT_ANTI)
 				cheapest_safe_inner = NULL;
 			else if (cheapest_total_inner->parallel_safe)
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 0e82ca1867a..d10095de70f 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -3080,33 +3080,6 @@ select * from tbl_rs t1 join
  3 | 3 | 4 | 4
 (6 rows)
 
---
--- regression test for bug with parallel-hash-right-semi join
---
-begin;
--- encourage use of parallel plans
-set local parallel_setup_cost=0;
-set local parallel_tuple_cost=0;
-set local min_parallel_table_scan_size=0;
-set local max_parallel_workers_per_gather=4;
--- ensure we don't get parallel hash right semi join
-explain (costs off)
-select * from tenk1 t1
-where exists (select 1 from tenk1 t2 where fivethous = t1.fivethous)
-and t1.fivethous < 5;
-                    QUERY PLAN                    
---------------------------------------------------
- Gather
-   Workers Planned: 4
-   ->  Parallel Hash Semi Join
-         Hash Cond: (t1.fivethous = t2.fivethous)
-         ->  Parallel Seq Scan on tenk1 t1
-               Filter: (fivethous < 5)
-         ->  Parallel Hash
-               ->  Parallel Seq Scan on tenk1 t2
-(8 rows)
-
-rollback;
 --
 -- regression test for bug #13908 (hash join with skew tuples & nbatch increase)
 --
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index c6b8b09a381..b1732453e8d 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -759,26 +759,6 @@ select * from tbl_rs t1 join
             (select t1.a+t3.a from tbl_rs t3) and t2.a < 5)
   on true;
 
---
--- regression test for bug with parallel-hash-right-semi join
---
-
-begin;
-
--- encourage use of parallel plans
-set local parallel_setup_cost=0;
-set local parallel_tuple_cost=0;
-set local min_parallel_table_scan_size=0;
-set local max_parallel_workers_per_gather=4;
-
--- ensure we don't get parallel hash right semi join
-explain (costs off)
-select * from tenk1 t1
-where exists (select 1 from tenk1 t2 where fivethous = t1.fivethous)
-and t1.fivethous < 5;
-
-rollback;
-
 --
 -- regression test for bug #13908 (hash join with skew tuples & nbatch increase)
 --
-- 
2.51.1