Right, this issue has escalated from being a odd curiosity to having cost us a fortune in lost bookings.

Since roughly 4am our callcentre agents have received this error onscreen every time they try to complete a booking: Slony-I: old key column Booking."GuestID" IS NULL on UPDATE

Similarly to my previous research with DisplayNumberRooms, 

joltpg1:~# grep GuestID schema
    "GuestID" integer,

i.e. GuestID does not appear anywhere in the schema but its definition as part of the Booking table. The table has 600k rows, with 400k having NULL GuestID (we only introduced the concept of a GuestID in March).

I tried REINDEX on the Booking table, which made no difference. Temporarily I have just done SET DROP TABLE for Booking so that our core business function can continue, but I am at a loss as to what to try next.

It can't just be a coincidence that the nightly VACUUM VERBOSE ANALYSE finished running about 10 minutes before, but I cannot find anything pertinent in the log output to indicate any error condition.

Any ideas?

Cheers,
Gavin.