http://developer.postgresql.org/pgdocs/postgres/functions-info.html
xip_list is described as
"Active txids at the time of the snapshot... "
This is incorrect. The xip_list is the list of transactions that are in
progress *and* less than xmax. There may be transactions in progress
with an xid higher than xmax. This will happen frequently in fact. This
is because xmax is defined as the highest/latest completed xid, not the
highest running xid.
Note that there is no way to discover the list of running xids at the
time of the snapshot, from the data we hold about snapshots. Nor can the
snapshot data be used to monitor the number of transactions in progress.
Anyone disagree? If not, I'll patch.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support