xact_start for walsender & logical decoding not updated

[Tomas Vondra]

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 
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);