The PostgreSQL developers have discovered a bug in all released versions of pg_upgrade and (formerly) pg_migrator. Anyone who has used pg_upgrade or pg_migrator should take corrective actions as soon as possible.
You might also want to make a backup of the pg_clog directory if you do not already have a recent copy. If you fail to take action promptly, it might result in unexpected downtime.
This bug can cause queries to return the following error:
ERROR: could not access status of transaction ######
DETAIL: could not open file "pg_clog/####": No such file or directory=20
This error prevents access to very wide values stored in the database. To prevent such failures users need to follow the instructions on the pg_upgrade fix wiki page as soon as possible.
The PostgreSQL developers apologize for any disruption which this issue may cause.
Full information on the wiki page.
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