Set relfrozenxid to oldest extant XID seen by VACUUM.

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 <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CAH2-WzkymFbz6D_vL+jmqSn_5q1wsFvFrE+37yLgL_Rkfd6Gzg@mail.gmail.com

1 files changed, 4 insertions, 2 deletions

diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index b46ab7d7390..4403f01e130 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -167,8 +167,10 @@ extern void heap_inplace_update(Relation relation, HeapTuple tuple);
 extern bool heap_freeze_tuple(HeapTupleHeader tuple,
 							  TransactionId relfrozenxid, TransactionId relminmxid,
 							  TransactionId cutoff_xid, TransactionId cutoff_multi);
-extern bool heap_tuple_needs_freeze(HeapTupleHeader tuple, TransactionId cutoff_xid,
-									MultiXactId cutoff_multi);
+extern bool heap_tuple_would_freeze(HeapTupleHeader tuple, TransactionId cutoff_xid,
+									MultiXactId cutoff_multi,
+									TransactionId *relfrozenxid_out,
+									MultiXactId *relminmxid_out);
 extern bool heap_tuple_needs_eventual_freeze(HeapTupleHeader tuple);
 
 extern void simple_heap_insert(Relation relation, HeapTuple tup);