xact_start for walsender & logical decoding not updated

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