adding a new column in IDENTIFY_SYSTEM

Hi

I want to propose the addition of a new field in IDENTIFY_SYSTEM:
xlogversion, which will carry XLOG_PAGE_MAGIC from primary.
The idea of sending that info is to allow us to know if the xlog page
version of two different major versions are compatible or not.
Currently pg_upgrade requires the primary to be taken down, and then
re-run all base backups for each standby which is a lot of work and
doesn't sound like "online upgrade".

I want to add the field now to make the protocol stable, also because
when we connect to start replication we check for the number of fields
retrieved from IDENTIFY_SYSTEM so if we add it in 9.2 we will be
unable to do this until 9.3 (when both releases agree about the number
of fields returned).

patch is very simple and doesn't affect anyone nor this will require
an initdb so i guess is safe to apply now.

comments?

--
Jaime Casanova         www.2ndQuadrant.com
Professional PostgreSQL: Soporte y capacitación de PostgreSQL