Allow pg_class xid & multixid horizons to not be set.

This allows table AMs that don't need these horizons. This was already
documented in the tableam relation_set_new_filenode callback, but an
assert prevented if from actually working (the test AM code contained
the change itself). Defang the asserts in the general code, and move
the stronger ones into heap AM.

Relatedly, after CLUSTER/VACUUM, we'd always assign a relfrozenxid /
relminmxid. Change the table_relation_copy_for_cluster() interface to
allow the AM to overwrite the horizons that get set on the pg_class
entry.  This'd also in the future allow AMs like heap to compute a
relfrozenxid during rewrite that's the table's actual minimum rather
than a pre-determined value.  Arguably it'd have been better to move
the whole computation / setting of those values into the callback, but
it seems likely that for other reasons it'd be better to be able to
use one value to vacuum/cluster multiple tables (e.g. a toast's
horizon shouldn't be different than the table's).

Reported-By: Heikki Linnakangas
Author: Andres Freund
Discussion: https://postgr.es/m/9a7fb9cc-2419-5db7-8840-ddc10c93f122@iki.fi

src/backend/access/heap/heapam_handler.c

@@ -668,8 +668,8 @@ static void
 heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 								 Relation OldIndex, bool use_sort,
 								 TransactionId OldestXmin,
-								 TransactionId FreezeXid,
-								 MultiXactId MultiXactCutoff,
+								 TransactionId *xid_cutoff,
+								 MultiXactId *multi_cutoff,
 								 double *num_tuples,
 								 double *tups_vacuumed,
 								 double *tups_recently_dead)
@@ -707,8 +707,8 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 	isnull = (bool *) palloc(natts * sizeof(bool));
 
 	/* Initialize the rewrite operation */
-	rwstate = begin_heap_rewrite(OldHeap, NewHeap, OldestXmin, FreezeXid,
-								 MultiXactCutoff, use_wal);
+	rwstate = begin_heap_rewrite(OldHeap, NewHeap, OldestXmin, *xid_cutoff,
+								 *multi_cutoff, use_wal);
 
 
 	/* Set up sorting if wanted */

src/backend/access/heap/vacuumlazy.c

@@ -213,6 +213,10 @@ heap_vacuum_rel(Relation onerel, VacuumParams *params,
 	Assert(params != NULL);
 	Assert(params->index_cleanup != VACOPT_TERNARY_DEFAULT);
 
+	/* not every AM requires these to be valid, but heap does */
+	Assert(TransactionIdIsNormal(onerel->rd_rel->relfrozenxid));
+	Assert(MultiXactIdIsValid(onerel->rd_rel->relminmxid));
+
 	/* measure elapsed time iff autovacuum logging requires it */
 	if (IsAutoVacuumWorkerProcess() && params->log_min_duration >= 0)
 	{

src/backend/commands/cluster.c

@@ -870,19 +870,17 @@ copy_table_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 	 * FreezeXid will become the table's new relfrozenxid, and that mustn't go
 	 * backwards, so take the max.
 	 */
-	if (TransactionIdPrecedes(FreezeXid, OldHeap->rd_rel->relfrozenxid))
+	if (TransactionIdIsValid(OldHeap->rd_rel->relfrozenxid) &&
+		TransactionIdPrecedes(FreezeXid, OldHeap->rd_rel->relfrozenxid))
 		FreezeXid = OldHeap->rd_rel->relfrozenxid;
 
 	/*
 	 * MultiXactCutoff, similarly, shouldn't go backwards either.
 	 */
-	if (MultiXactIdPrecedes(MultiXactCutoff, OldHeap->rd_rel->relminmxid))
+	if (MultiXactIdIsValid(OldHeap->rd_rel->relminmxid) &&
+		MultiXactIdPrecedes(MultiXactCutoff, OldHeap->rd_rel->relminmxid))
 		MultiXactCutoff = OldHeap->rd_rel->relminmxid;
 
-	/* return selected values to caller */
-	*pFreezeXid = FreezeXid;
-	*pCutoffMulti = MultiXactCutoff;
-
 	/*
 	 * Decide whether to use an indexscan or seqscan-and-optional-sort to scan
 	 * the OldHeap.  We know how to use a sort to duplicate the ordering of a
@@ -915,13 +913,19 @@ copy_table_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 
 	/*
 	 * Hand of the actual copying to AM specific function, the generic code
-	 * cannot know how to deal with visibility across AMs.
+	 * cannot know how to deal with visibility across AMs. Note that this
+	 * routine is allowed to set FreezeXid / MultiXactCutoff to different
+	 * values (e.g. because the AM doesn't use freezing).
 	 */
 	table_relation_copy_for_cluster(OldHeap, NewHeap, OldIndex, use_sort,
-									OldestXmin, FreezeXid, MultiXactCutoff,
+									OldestXmin, &FreezeXid, &MultiXactCutoff,
 									&num_tuples, &tups_vacuumed,
 									&tups_recently_dead);
 
+	/* return selected values to caller, get set as relfrozenxid/minmxid */
+	*pFreezeXid = FreezeXid;
+	*pCutoffMulti = MultiXactCutoff;
+
 	/* Reset rd_toastoid just to be tidy --- it shouldn't be looked at again */
 	NewHeap->rd_toastoid = InvalidOid;
 
@@ -1118,9 +1122,9 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class,
 	/* set rel1's frozen Xid and minimum MultiXid */
 	if (relform1->relkind != RELKIND_INDEX)
 	{
-		Assert(TransactionIdIsNormal(frozenXid));
+		Assert(!TransactionIdIsValid(frozenXid) ||
+			   TransactionIdIsNormal(frozenXid));
 		relform1->relfrozenxid = frozenXid;
-		Assert(MultiXactIdIsValid(cutoffMulti));
 		relform1->relminmxid = cutoffMulti;
 	}
 

src/backend/commands/vacuum.c

@@ -1313,36 +1313,61 @@ vac_update_datfrozenxid(void)
 
 		/*
 		 * Only consider relations able to hold unfrozen XIDs (anything else
-		 * should have InvalidTransactionId in relfrozenxid anyway.)
+		 * should have InvalidTransactionId in relfrozenxid anyway).
 		 */
 		if (classForm->relkind != RELKIND_RELATION &&
 			classForm->relkind != RELKIND_MATVIEW &&
 			classForm->relkind != RELKIND_TOASTVALUE)
+		{
+			Assert(!TransactionIdIsValid(classForm->relfrozenxid));
+			Assert(!MultiXactIdIsValid(classForm->relminmxid));
 			continue;
-
-		Assert(TransactionIdIsNormal(classForm->relfrozenxid));
-		Assert(MultiXactIdIsValid(classForm->relminmxid));
+		}
 
 		/*
+		 * Some table AMs might not need per-relation xid / multixid
+		 * horizons. It therefore seems reasonable to allow relfrozenxid and
+		 * relminmxid to not be set (i.e. set to their respective Invalid*Id)
+		 * independently. Thus validate and compute horizon for each only if
+		 * set.
+		 *
 		 * If things are working properly, no relation should have a
 		 * relfrozenxid or relminmxid that is "in the future".  However, such
 		 * cases have been known to arise due to bugs in pg_upgrade.  If we
 		 * see any entries that are "in the future", chicken out and don't do
-		 * anything.  This ensures we won't truncate clog before those
-		 * relations have been scanned and cleaned up.
+		 * anything.  This ensures we won't truncate clog & multixact SLRUs
+		 * before those relations have been scanned and cleaned up.
 		 */
-		if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid) ||
-			MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))
+
+		if (TransactionIdIsValid(classForm->relfrozenxid))
 		{
-			bogus = true;
-			break;
+			Assert(TransactionIdIsNormal(classForm->relfrozenxid));
+
+			/* check for values in the future */
+			if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid))
+			{
+				bogus = true;
+				break;
+			}
+
+			/* determine new horizon */
+			if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))
+				newFrozenXid = classForm->relfrozenxid;
 		}
 
-		if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))
-			newFrozenXid = classForm->relfrozenxid;
+		if (MultiXactIdIsValid(classForm->relminmxid))
+		{
+			/* check for values in the future */
+			if (MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))
+			{
+				bogus = true;
+				break;
+			}
 
-		if (MultiXactIdPrecedes(classForm->relminmxid, newMinMulti))
-			newMinMulti = classForm->relminmxid;
+			/* determine new horizon */
+			if (MultiXactIdPrecedes(classForm->relminmxid, newMinMulti))
+				newMinMulti = classForm->relminmxid;
+		}
 	}
 
 	/* we're done with pg_class */

src/backend/postmaster/autovacuum.c

@@ -3033,8 +3033,8 @@ relation_needs_vacanalyze(Oid relid,
 		multiForceLimit = recentMulti - multixact_freeze_max_age;
 		if (multiForceLimit < FirstMultiXactId)
 			multiForceLimit -= FirstMultiXactId;
-		force_vacuum = MultiXactIdPrecedes(classForm->relminmxid,
-										   multiForceLimit);
+		force_vacuum = MultiXactIdIsValid(classForm->relminmxid) &&
+			MultiXactIdPrecedes(classForm->relminmxid, multiForceLimit);
 	}
 	*wraparound = force_vacuum;
 

src/include/access/tableam.h

@@ -452,8 +452,8 @@ typedef struct TableAmRoutine
 											  Relation OldIndex,
 											  bool use_sort,
 											  TransactionId OldestXmin,
-											  TransactionId FreezeXid,
-											  MultiXactId MultiXactCutoff,
+											  TransactionId *xid_cutoff,
+											  MultiXactId *multi_cutoff,
 											  double *num_tuples,
 											  double *tups_vacuumed,
 											  double *tups_recently_dead);
@@ -1297,32 +1297,37 @@ table_relation_copy_data(Relation rel, RelFileNode newrnode)
  * Copy data from `OldHeap` into `NewHeap`, as part of a CLUSTER or VACUUM
  * FULL.
  *
- * If `use_sort` is true, the table contents are sorted appropriate for
- * `OldIndex`; if use_sort is false and OldIndex is not InvalidOid, the data
- * is copied in that index's order; if use_sort is false and OidIndex is
- * InvalidOid, no sorting is performed.
+ * Additional Input parameters:
+ * - use_sort - if true, the table contents are sorted appropriate for
+ *   `OldIndex`; if false and OldIndex is not InvalidOid, the data is copied
+ *   in that index's order; if false and OidIndex is InvalidOid, no sorting is
+ *   performed
+ * - OidIndex - see use_sort
+ * - OldestXmin - computed by vacuum_set_xid_limits(), even when
+ *   not needed for the relation's AM
+ * - *xid_cutoff - dito
+ * - *multi_cutoff - dito
  *
- * OldestXmin, FreezeXid, MultiXactCutoff must be currently valid values for
- * the table.
- *
- * *num_tuples, *tups_vacuumed, *tups_recently_dead will contain statistics
- * computed while copying for the relation. Not all might make sense for every
- * AM.
+ * Output parameters:
+ * - *xid_cutoff - rel's new relfrozenxid value, may be invalid
+ * - *multi_cutoff - rel's new relminmxid value, may be invalid
+ * - *tups_vacuumed - stats, for logging, if appropriate for AM
+ * - *tups_recently_dead - stats, for logging, if appropriate for AM
  */
 static inline void
 table_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 								Relation OldIndex,
 								bool use_sort,
 								TransactionId OldestXmin,
-								TransactionId FreezeXid,
-								MultiXactId MultiXactCutoff,
+								TransactionId *xid_cutoff,
+								MultiXactId *multi_cutoff,
 								double *num_tuples,
 								double *tups_vacuumed,
 								double *tups_recently_dead)
 {
 	OldHeap->rd_tableam->relation_copy_for_cluster(OldHeap, NewHeap, OldIndex,
 												   use_sort, OldestXmin,
-												   FreezeXid, MultiXactCutoff,
+												   xid_cutoff, multi_cutoff,
 												   num_tuples, tups_vacuumed,
 												   tups_recently_dead);
 }