2) PGSTAT_BACKEND_FLUSH_XACT collision with newly committed PGSTAT_BACKEND_FLUSH_LOCK.
3) What if pgStatXactCommit and pgStatXactRollback are 0? We unnecessarily make backend_has_xactstats as true while updating the pending struct values. In this process unnecessary ref_lock would be taken and stats.xact_commit, stats.xact_rollback would be updated with 0 and then ref_lock is unlocked.
4) PendingBackendStats was static but it was removed in this change, yet the comment above still mentions it. Also it is killing the purpose of "Utility routines to report I/O stats for backends ..." comment mentioned in pgstat_backend.c. It appears the existing helper I/O routines' purpose is eliminated. This doesn't look good.
4) I don't see any function using plural form like pg_stat_get_backend_transactions in pg_proc.dat
5) The thread started with only commit and rollback counters, but xid was added in v3, along with the idea of adding more, such as "seqscans, vacuum count, analyze count...". This makes me curious: why can't these other counters be added in this same thread? If these others aren't added now, why was xid added? I suggest this thread should only include commit and rollback counters, and xid should be added with the other metrics in a new thread.
