If there's a lesson for the project in this, it's "don't wrap an update release the same week as Feature Freeze".
Anyway, given this it would be good for users to determine more definitively if they are in actual danger of Multixact member wraparound (MMW hereafter), so that they know if they need to apply the update right away despite issues. Here's how to do that:
- navigate to the PostgreSQL data directory on your server(s), e.g. "/var/lib/postgresql/9.3/main"
- switch to the subdirectory "pg_multixact/members"
- count the number of files in this directory, e.g. "ls -l | wc -l"
Of course, there are other fixes in this latest update, and if one of them specifically affects you, you may have updated already.
Thank you to Thomas Munro for pointing out the simple way to determine this and giving me background material on the fixes.
Note: the above is my personal advice, and has not been approved by the PostgreSQL project, core team, or PostgreSQL Experts Inc. The PostgreSQL project generally advises applying all updates promptly.