Date: Tue, 10 Dec 2019 00:44:09 +0100
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: pgsql-hackers@postgresql.org
Subject: xact_start for walsender & logical decoding not updated
Message-ID: <20191209234409.exe7osmyalwkt5j4@development>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="7c3xbwnoh322btiw"
Content-Disposition: inline
Precedence: bulk


--7c3xbwnoh322btiw
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Disposition: inline

Hi,

I think there's a minor bug in pg_stat_activity tracking of walsender
processes. The issue is that xact_start is only updated at the very
beginning when the walsender starts (so it's almost exactly equal to
backend_start) and then just flips between NULL and that value.

Reproducing this is trivial - just create a publication/subscription
with the built-in logical replication, and run arbitrary workload.
You'll see that the xact_start value never changes.

I think the right fix is calling SetCurrentStatementStartTimestamp()
right before StartTransactionCommand() in ReorderBufferCommit, per the
attached patch.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services 

--7c3xbwnoh322btiw
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment;
	filename="reorderbuffer-timestamp-fix.patch"

diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index 53affeb877..5235fb31b8 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -1554,7 +1554,10 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 		if (using_subtxn)
 			BeginInternalSubTransaction("replay");
 		else
+		{
+			SetCurrentStatementStartTimestamp();
 			StartTransactionCommand();
+		}
 
 		rb->begin(rb, txn);
 

--7c3xbwnoh322btiw--