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From: Zhijie Hou (Fujitsu) <[email protected]>
To: PostgreSQL Hackers <[email protected]>
Subject: Fix stats reporting delays in logical parallel apply worker
Date: Fri, 17 Apr 2026 03:01:08 +0000
Message-ID: <TYRPR01MB1419579F217CC4332B615589594202@TYRPR01MB14195.jpnprd01.prod.outlook.com> (raw)

Hi,

When implementing another feature, I noticed that parallel apply workers
currently do not report statistics while idle in their main loop. This can cause
stats from the last processed transaction to be arbitrarily delayed, especially
when there are long gaps between streamed transactions.

The issue is demonstrated in 0002, where a TAP test fails when attempting to
collect stats from a parallel apply worker that has no subsequent transaction to
trigger a stats report.

0001 fixes this issue by forcing a stats report when the worker is idle in the
main loop, matching the behavior already present in LogicalRepApplyLoop() for
regular logical apply workers.

Best Regards,
Hou zj


Attachments:

  [application/octet-stream] v1-0002-Test-the-stats-report-in-parallel-apply-worker.patch (1.3K, 2-v1-0002-Test-the-stats-report-in-parallel-apply-worker.patch)
  download | inline diff:
From 0fa996424feeb17032bf44e2c5d5bbb44d65228b Mon Sep 17 00:00:00 2001
From: Zhijie Hou <[email protected]>
Date: Wed, 15 Apr 2026 16:59:53 +0800
Subject: [PATCH v1 2/2] Test the stats report in parallel apply worker

---
 src/test/subscription/t/026_stats.pl | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/test/subscription/t/026_stats.pl b/src/test/subscription/t/026_stats.pl
index 5d457060a02..178b3b71e49 100644
--- a/src/test/subscription/t/026_stats.pl
+++ b/src/test/subscription/t/026_stats.pl
@@ -136,6 +136,15 @@ sub create_sub_pub_w_errors
 	# Truncate test table so that apply worker can continue.
 	$node_subscriber->safe_psql($db, qq(TRUNCATE $table_name));
 
+	# Force the publisher to stream the changes to the subscriber immediately so
+	# that the delete_missing conflict can be tested in the parallel apply
+	# worker.
+	$node_publisher->append_conf('postgresql.conf',
+		"debug_logical_replication_streaming = immediate");
+	$node_publisher->reload;
+
+	$node_subscriber->safe_psql($db, qq(ALTER SUBSCRIPTION $sub_name SET (streaming = parallel)));
+
 	# Delete data from the test table on the publisher. This delete operation
 	# should be skipped on the subscriber since the table is already empty.
 	$node_publisher->safe_psql($db, qq(DELETE FROM $table_name;));
-- 
2.53.0.windows.2



  [application/octet-stream] v1-0001-Fix-stats-reporting-delays-in-parallel-apply-work.patch (1.4K, 3-v1-0001-Fix-stats-reporting-delays-in-parallel-apply-work.patch)
  download | inline diff:
From d6df6255fdc49e26e65637826358de2a7ce37bc1 Mon Sep 17 00:00:00 2001
From: Zhijie Hou <[email protected]>
Date: Wed, 15 Apr 2026 17:30:03 +0800
Subject: [PATCH v1 1/2] Fix stats reporting delays in parallel apply worker

Parallel apply workers currently do not flush statistics while idle in their
main loop. This can cause stats from the last processed transaction to be
arbitrarily delayed, especially when there are long gaps between streamed
transactions.

Fix this by forcing a stats report when the worker is idle in the main loop,
ensuring timely visibility of accumulated statistics.
---
 src/backend/replication/logical/applyparallelworker.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/backend/replication/logical/applyparallelworker.c b/src/backend/replication/logical/applyparallelworker.c
index 798e8d85b3e..aaa21a5a94a 100644
--- a/src/backend/replication/logical/applyparallelworker.c
+++ b/src/backend/replication/logical/applyparallelworker.c
@@ -815,6 +815,15 @@ LogicalParallelApplyLoop(shm_mq_handle *mqh)
 
 				if (rc & WL_LATCH_SET)
 					ResetLatch(MyLatch);
+
+				/*
+				 * Force stats reporting to avoid long delays. There can be long
+				 * idle gaps before the leader assigns the next transaction, and
+				 * the only opportunity to report stats during such gaps is
+				 * here.
+				 */
+				if ((rc & WL_TIMEOUT) && !IsTransactionState())
+					pgstat_report_stat(true);
 			}
 		}
 		else
-- 
2.53.0.windows.2

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