Patch: show xid and xmin in pg_stat_activity and pg_stat_replication

Hi,

attached you will find a patch for showing the current transaction id
(xid) and the xmin of a backend in pg_stat_activty and the xmin in
pg_stat_replication.

This may be helpful when looking for the cause of bloat.

I added two new struct members in PgBackendStatus which get filled in
pgstat_read_current_status() and slightly modified the catalog schema
and the pg_stat_get_activity() procedure.

I'm not sure if it is a good idea to gather the data in
pgstat_read_current_status(), but I chose to do it this way
nonetheless because otherwise I would have to create collector
functions like pgstat_report_xid_assignment() /
pgstat_report_xmin_changed() (accordingly to
pgstat_report_xact_timestamp()) which may result in a performance hit.

Regards,

--
 Christian Kruse               http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services