When a fail over occurs, all of existing client sessions will be
killed because all child process are forced to restart. However in
certain cases we do not need to restart all of the child process. For
now on, we assume pgpool-II operates in streaming replication mode and
the PostgreSQL server going down is a standby server.

case 1: pcp_detach_node

pcp_detach_node is a command to request a switch over (the standby
server may or may not go down in this case). It is desirable that an
admin could use the command to make a standby server out of use
without disturbing existing sessions.

From pgpool-II 3.6, some of client sessions could survive if following
conditions are all met:

- the load balancing node assigned to the session is not the node in
  question.

- the node is not the primary.

When pcp_detach_node is executed, pgpool-II parent process checks the
load balancing node info on the shared memory called "ProcessInfo". If
pgpool-II child process associated with the session uses the load
balancing node, the pgpool-II parent sends a signal to the child
process to request exiting. Thus the session will be killed.  In other
cases, the session will survive. Suppose we have two DB nodes, a
primary and a standby. Chances for the session to survive will be 50%
assuming that the weight parameters are equal among the nodes.

In practice, following operations are recommended to avoid sudden
session killing.

- Change the weight parameter for the node scheduled to be down to 0 in
  pgpool.conf.

- Do "pgpool reload" to reflect the change to pgpool-II.

- Wait until all existing sessions to the node finish.

- Execute pcp_detach_node.

- Shutdown the node.

case 2: PostgreSQL shutdown