Remove vacuum_defer_cleanup_age

vacuum_defer_cleanup_age was introduced before hot_standby_feedback and
replication slots existed. It is hard to use reasonably - commonly it will
either be set too low (not preventing recovery conflicts, while still causing
some bloat), or too high (causing a lot of bloat). The alternatives do not
have that issue.

That on its own might not be sufficient reason to remove
vacuum_defer_cleanup_age, but it also complicates computation of xid
horizons. See e.g. the bug fixed in be504a3e97. It also is untested.

This commit removes TransactionIdRetreatSafely(), as there are no users
anymore. There might be potential future users, hence noting that here.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20230317230930.nhsgk3qfk7f4axls@awork3.anarazel.de

src/backend/storage/ipc/procarray.c

@@ -367,9 +367,6 @@ static inline void ProcArrayEndTransactionInternal(PGPROC *proc, TransactionId l
 static void ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid);
 static void MaintainLatestCompletedXid(TransactionId latestXid);
 static void MaintainLatestCompletedXidRecovery(TransactionId latestXid);
-static void TransactionIdRetreatSafely(TransactionId *xid,
-									   int retreat_by,
-									   FullTransactionId rel);
 
 static inline FullTransactionId FullXidRelativeTo(FullTransactionId rel,
 												  TransactionId xid);
@@ -1709,10 +1706,7 @@ TransactionIdIsActive(TransactionId xid)
  * do about that --- data is only protected if the walsender runs continuously
  * while queries are executed on the standby.  (The Hot Standby code deals
  * with such cases by failing standby queries that needed to access
- * already-removed data, so there's no integrity bug.)  The computed values
- * are also adjusted with vacuum_defer_cleanup_age, so increasing that setting
- * on the fly is another easy way to make horizons move backwards, with no
- * consequences for data integrity.
+ * already-removed data, so there's no integrity bug.)
  *
  * Note: the approximate horizons (see definition of GlobalVisState) are
  * updated by the computations done here. That's currently required for
@@ -1877,50 +1871,11 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 			TransactionIdOlder(h->data_oldest_nonremovable, kaxmin);
 		/* temp relations cannot be accessed in recovery */
 	}
-	else
-	{
-		/*
-		 * Compute the cutoff XID by subtracting vacuum_defer_cleanup_age.
-		 *
-		 * vacuum_defer_cleanup_age provides some additional "slop" for the
-		 * benefit of hot standby queries on standby servers.  This is quick
-		 * and dirty, and perhaps not all that useful unless the primary has a
-		 * predictable transaction rate, but it offers some protection when
-		 * there's no walsender connection.  Note that we are assuming
-		 * vacuum_defer_cleanup_age isn't large enough to cause wraparound ---
-		 * so guc.c should limit it to no more than the xidStopLimit threshold
-		 * in varsup.c.  Also note that we intentionally don't apply
-		 * vacuum_defer_cleanup_age on standby servers.
-		 *
-		 * Need to use TransactionIdRetreatSafely() instead of open-coding the
-		 * subtraction, to prevent creating an xid before
-		 * FirstNormalTransactionId.
-		 */
-		Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
-											 h->shared_oldest_nonremovable));
-		Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
-											 h->data_oldest_nonremovable));
 
-		if (vacuum_defer_cleanup_age > 0)
-		{
-			TransactionIdRetreatSafely(&h->oldest_considered_running,
-									   vacuum_defer_cleanup_age,
-									   h->latest_completed);
-			TransactionIdRetreatSafely(&h->shared_oldest_nonremovable,
-									   vacuum_defer_cleanup_age,
-									   h->latest_completed);
-			TransactionIdRetreatSafely(&h->data_oldest_nonremovable,
-									   vacuum_defer_cleanup_age,
-									   h->latest_completed);
-			/* defer doesn't apply to temp relations */
-
-
-			Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
-												 h->shared_oldest_nonremovable));
-			Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
-												 h->data_oldest_nonremovable));
-		}
-	}
+	Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
+										 h->shared_oldest_nonremovable));
+	Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
+										 h->data_oldest_nonremovable));
 
 	/*
 	 * Check whether there are replication slots requiring an older xmin.
@@ -1947,8 +1902,8 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 						   h->slot_catalog_xmin);
 
 	/*
-	 * It's possible that slots / vacuum_defer_cleanup_age backed up the
-	 * horizons further than oldest_considered_running. Fix.
+	 * It's possible that slots backed up the horizons further than
+	 * oldest_considered_running. Fix.
 	 */
 	h->oldest_considered_running =
 		TransactionIdOlder(h->oldest_considered_running,
@@ -2490,15 +2445,9 @@ GetSnapshotData(Snapshot snapshot)
 		 */
 		oldestfxid = FullXidRelativeTo(latest_completed, oldestxid);
 
-		/* apply vacuum_defer_cleanup_age */
-		def_vis_xid_data = xmin;
-		TransactionIdRetreatSafely(&def_vis_xid_data,
-								   vacuum_defer_cleanup_age,
-								   oldestfxid);
-
 		/* Check whether there's a replication slot requiring an older xmin. */
 		def_vis_xid_data =
-			TransactionIdOlder(def_vis_xid_data, replication_slot_xmin);
+			TransactionIdOlder(xmin, replication_slot_xmin);
 
 		/*
 		 * Rows in non-shared, non-catalog tables possibly could be vacuumed
@@ -4320,44 +4269,6 @@ GlobalVisCheckRemovableXid(Relation rel, TransactionId xid)
 	return GlobalVisTestIsRemovableXid(state, xid);
 }
 
-/*
- * Safely retract *xid by retreat_by, store the result in *xid.
- *
- * Need to be careful to prevent *xid from retreating below
- * FirstNormalTransactionId during epoch 0. This is important to prevent
- * generating xids that cannot be converted to a FullTransactionId without
- * wrapping around.
- *
- * If retreat_by would lead to a too old xid, FirstNormalTransactionId is
- * returned instead.
- */
-static void
-TransactionIdRetreatSafely(TransactionId *xid, int retreat_by, FullTransactionId rel)
-{
-	TransactionId original_xid = *xid;
-	FullTransactionId fxid;
-	uint64		fxid_i;
-
-	Assert(TransactionIdIsNormal(original_xid));
-	Assert(retreat_by >= 0);	/* relevant GUCs are stored as ints */
-	AssertTransactionIdInAllowableRange(original_xid);
-
-	if (retreat_by == 0)
-		return;
-
-	fxid = FullXidRelativeTo(rel, original_xid);
-	fxid_i = U64FromFullTransactionId(fxid);
-
-	if ((fxid_i - FirstNormalTransactionId) <= retreat_by)
-		*xid = FirstNormalTransactionId;
-	else
-	{
-		*xid = TransactionIdRetreatedBy(original_xid, retreat_by);
-		Assert(TransactionIdIsNormal(*xid));
-		Assert(NormalTransactionIdPrecedes(*xid, original_xid));
-	}
-}
-
 /*
  * Convert a 32 bit transaction id into 64 bit transaction id, by assuming it
  * is within MaxTransactionId / 2 of XidFromFullTransactionId(rel).

src/backend/storage/ipc/standby.c

@@ -38,7 +38,6 @@
 #include "utils/timestamp.h"
 
 /* User-settable GUC parameters */
-int			vacuum_defer_cleanup_age;
 int			max_standby_archive_delay = 30 * 1000;
 int			max_standby_streaming_delay = 30 * 1000;
 bool		log_recovery_conflict_waits = false;