When VACUUM set relfrozenxid before now, it set it to whatever value was used to determine which tuples to freeze -- the FreezeLimit cutoff. This approach was very naive. The relfrozenxid invariant only requires that new relfrozenxid values be <= the oldest extant XID remaining in the table (at the point that the VACUUM operation ends), which in general might be much more recent than FreezeLimit.
VACUUM now carefully tracks the oldest remaining XID/MultiXactId as it goes (the oldest remaining values _after_ lazy_scan_prune processing). The final values are set as the table's new relfrozenxid and new relminmxid in pg_class at the end of each VACUUM. The oldest XID might come from a tuple's xmin, xmax, or xvac fields. It might even come from one of the table's remaining MultiXacts.
Final relfrozenxid values must still be >= FreezeLimit in an aggressive
VACUUM (FreezeLimit still acts as a lower bound on the final value that aggressive VACUUM can set relfrozenxid to). Since standard VACUUMs still make no guarantees about advancing relfrozenxid, they might as well set relfrozenxid to a value from well before FreezeLimit when the opportunity presents itself. In general standard VACUUMs may now set relfrozenxid to any value > the original relfrozenxid and <= OldestXmin.
Credit for the general idea of using the oldest extant XID to set pg_class.relfrozenxid at the end of VACUUM goes to Andres Freund.
Author: Peter Geoghegan
0b018fabaa Set relfrozenxid to oldest extant XID seen by VACUUM.
doc/src/sgml/maintenance.sgml | 16 +-
src/backend/access/heap/heapam.c | 332 +++++++++++++++------
src/backend/access/heap/vacuumlazy.c | 156 ++++++----
src/backend/commands/cluster.c | 11 +-
src/backend/commands/vacuum.c | 39 ++-
src/include/access/heapam.h | 6 +-
src/include/access/heapam_xlog.h | 4 +-
src/include/commands/vacuum.h | 1 +
.../isolation/expected/vacuum-no-cleanup-lock.out | 189 ++++++++++++
src/test/isolation/expected/vacuum-reltuples.out | 67 -----
src/test/isolation/isolation_schedule | 2 +-
.../isolation/specs/vacuum-no-cleanup-lock.spec | 150 ++++++++++
src/test/isolation/specs/vacuum-reltuples.spec | 49 ---
13 files changed, 717 insertions(+), 305 deletions(-)