Richard Huxton <dev@archonet.com> writes:
> Well, both cluster and vacuum full will require a lock on the table. But
> they're more or less doing the same thing, so why the one should work
> and the other not I don't know.
CLUSTER isn't MVCC-safe --- it'll drop rows that are committed dead
even if they are potentially still visible to some open transaction.
The OP has *clearly* got an open-transaction problem.
regards, tom lane