Re: BUG #17949: Adding an index introduces serialisation anomalies.

How is this schedule supposed to work?

S1:  _bt_search(&buf)
S1:  if (!BufferIsValid(buf)) // because index is empty
S1:  {
S1:    PredicateLockRelation(...);
S1:    ...
S1:    return false; // no tuples for you

S2:  _bt_search(&buf)

S1:  INSERT ...

S2:  if (!BufferIsValid(buf)) // because index *was* empty
S2:  {
S2:    PredicateLockRelation(...);
S2:    ...
S2:    return false; // no tuples for you

S2:  ...

My point is that S2 won't ever scan S1's tuples, so it won't pass S1's
xid to CheckForSerializableConflictOut().  Am I missing something?  I
can repro this with NR_THREADS set to only 2, after inserting
pg_usleep(1) after the _bt_search() in _bt_first() (sched_yield()
wasn't quite enough).