From 9b820630c522d3d8beebe647f0410e88c4b8730f Mon Sep 17 00:00:00 2001
From: Tender Wang <tndrwang@gmail.com>
Date: Fri, 6 Nov 2026 17:38:33 +0800
Subject: [PATCH] Fix MERGE insert counts when a trigger suppresses insertion.

When a BEFORE INSERT trigger returns NULL, ExecInsert() skips the
row, but its MERGE caller still increments mt_merge_inserted.
EXPLAIN ANALYZE consequently reports the row as inserted instead
of skipped.

Count MERGE INSERT actions in ExecInsert() after the row is inserted
or queued for FDW batch insertion. Check the current MERGE action
so that the insert half of a cross-partition UPDATE is excluded
from the INSERT count.
---
 src/backend/executor/nodeModifyTable.c |  9 ++++++++-
 src/test/regress/expected/merge.out    | 20 ++++++++++++++++++++
 src/test/regress/sql/merge.sql         | 17 +++++++++++++++++
 3 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index 558ceece9bb..62cf06ae4a0 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -879,6 +879,8 @@ ExecInsert(ModifyTableContext *context,
 	OnConflictAction onconflict = node->onConflictAction;
 	PartitionTupleRouting *proute = mtstate->mt_partition_tuple_routing;
 	MemoryContext oldContext;
+	bool		merge_insert = mtstate->operation == CMD_MERGE &&
+		mtstate->mt_merge_action->mas_action->commandType == CMD_INSERT;
 
 	/*
 	 * If the input result relation is a partitioned table, find the leaf
@@ -1033,6 +1035,10 @@ ExecInsert(ModifyTableContext *context,
 
 			resultRelInfo->ri_NumSlots++;
 
+			/* Count MERGE inserts queued for FDW batching. */
+			if (merge_insert)
+				mtstate->mt_merge_inserted++;
+
 			MemoryContextSwitchTo(oldContext);
 
 			return NULL;
@@ -1277,6 +1283,8 @@ ExecInsert(ModifyTableContext *context,
 
 	if (canSetTag)
 		(estate->es_processed)++;
+	if (merge_insert)
+		mtstate->mt_merge_inserted++;
 
 	/*
 	 * If this insert is the result of a partition key update that moved the
@@ -3895,7 +3903,6 @@ ExecMergeNotMatched(ModifyTableContext *context, ResultRelInfo *resultRelInfo,
 
 				rslot = ExecInsert(context, mtstate->rootResultRelInfo,
 								   newslot, canSetTag, NULL, NULL);
-				mtstate->mt_merge_inserted += 1;
 				break;
 			case CMD_NOTHING:
 				/* Do nothing */
diff --git a/src/test/regress/expected/merge.out b/src/test/regress/expected/merge.out
index 9cb1d87066a..0a1a181f4ee 100644
--- a/src/test/regress/expected/merge.out
+++ b/src/test/regress/expected/merge.out
@@ -1084,6 +1084,26 @@ SELECT * FROM target FULL OUTER JOIN source ON (sid = tid);
    1 |      10 |     |      
 (4 rows)
 
+-- A suppressed INSERT must be counted as skipped by EXPLAIN ANALYZE.
+DO $$
+DECLARE
+  plan json;
+BEGIN
+  EXECUTE $explain$
+    EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF, SUMMARY OFF, FORMAT JSON)
+    MERGE INTO target t USING (VALUES (4, 40)) AS s(sid, delta)
+    ON t.tid = s.sid
+    WHEN NOT MATCHED THEN INSERT VALUES (s.sid, s.delta)
+  $explain$ INTO plan;
+  RAISE NOTICE 'inserted = %, skipped = %',
+    plan->0->'Plan'->>'Tuples Inserted',
+    plan->0->'Plan'->>'Tuples Skipped';
+END;
+$$;
+NOTICE:  BEFORE INSERT STATEMENT trigger
+NOTICE:  BEFORE INSERT ROW trigger row: (4,40)
+NOTICE:  AFTER INSERT STATEMENT trigger
+NOTICE:  inserted = 0, skipped = 1
 DROP TRIGGER merge_skip ON target;
 DROP FUNCTION skip_merge_op();
 -- test from PL/pgSQL
diff --git a/src/test/regress/sql/merge.sql b/src/test/regress/sql/merge.sql
index 2660b19f238..20710805e36 100644
--- a/src/test/regress/sql/merge.sql
+++ b/src/test/regress/sql/merge.sql
@@ -713,6 +713,23 @@ RAISE NOTICE 'ROW_COUNT = %', result;
 END;
 $$;
 SELECT * FROM target FULL OUTER JOIN source ON (sid = tid);
+
+-- A suppressed INSERT must be counted as skipped by EXPLAIN ANALYZE.
+DO $$
+DECLARE
+  plan json;
+BEGIN
+  EXECUTE $explain$
+    EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF, SUMMARY OFF, FORMAT JSON)
+    MERGE INTO target t USING (VALUES (4, 40)) AS s(sid, delta)
+    ON t.tid = s.sid
+    WHEN NOT MATCHED THEN INSERT VALUES (s.sid, s.delta)
+  $explain$ INTO plan;
+  RAISE NOTICE 'inserted = %, skipped = %',
+    plan->0->'Plan'->>'Tuples Inserted',
+    plan->0->'Plan'->>'Tuples Skipped';
+END;
+$$;
 DROP TRIGGER merge_skip ON target;
 DROP FUNCTION skip_merge_op();
 
-- 
2.43.0