diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml
index a5271a9f8f2..965dcf472ca 100644
--- a/doc/src/sgml/ref/create_index.sgml
+++ b/doc/src/sgml/ref/create_index.sgml
@@ -855,6 +855,8 @@ Indexes:
    Like any long-running transaction, <command>CREATE INDEX</command> on a
    table can affect which tuples can be removed by concurrent
    <command>VACUUM</command> on any other table.
+   Excepted from this are operations with the <literal>CONCURRENTLY</literal>
+   option for indexes that are not partial and do not index any expressions.
   </para>
 
   <para>
diff --git a/doc/src/sgml/ref/reindex.sgml b/doc/src/sgml/ref/reindex.sgml
index 07795b57372..b22d39eba92 100644
--- a/doc/src/sgml/ref/reindex.sgml
+++ b/doc/src/sgml/ref/reindex.sgml
@@ -478,6 +478,8 @@ Indexes:
     Like any long-running transaction, <command>REINDEX</command> on a table
     can affect which tuples can be removed by concurrent
     <command>VACUUM</command> on any other table.
+    Excepted from this are operations with the <literal>CONCURRENTLY</literal>
+    option for indexes that are not partial and do not index any expressions.
    </para>
 
    <para>
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index a5daea89571..d736d06d288 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1610,7 +1610,13 @@ TransactionIdIsActive(TransactionId xid)
  * relations that's not required, since only backends in my own database could
  * ever see the tuples in them. Also, we can ignore concurrently running lazy
  * VACUUMs because (a) they must be working on other tables, and (b) they
- * don't need to do snapshot-based lookups.
+ * don't need to do snapshot-based lookups.  Similarly, for the non-catalog
+ * horizon, we can ignore CREATE INDEX CONCURRENTLY and REINDEX CONCURRENTLY
+ * when they are working on non-partial, non-expressional indexes, for the
+ * same reasons and because they can't run in transaction blocks.  (They are
+ * not possible to ignore for catalogs, because CIC and RC do some catalog
+ * operations.)  Do note that this means that CIC and RC must use a lock level
+ * that conflicts with VACUUM.
  *
  * This also computes a horizon used to truncate pg_subtrans. For that
  * backends in all databases have to be considered, and concurrently running
@@ -1660,9 +1666,6 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 	bool		in_recovery = RecoveryInProgress();
 	TransactionId *other_xids = ProcGlobal->xids;
 
-	/* inferred after ProcArrayLock is released */
-	h->catalog_oldest_nonremovable = InvalidTransactionId;
-
 	LWLockAcquire(ProcArrayLock, LW_SHARED);
 
 	h->latest_completed = ShmemVariableCache->latestCompletedXid;
@@ -1682,6 +1685,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 
 		h->oldest_considered_running = initial;
 		h->shared_oldest_nonremovable = initial;
+		h->catalog_oldest_nonremovable = initial;
 		h->data_oldest_nonremovable = initial;
 
 		/*
@@ -1752,7 +1756,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 		if (statusFlags & (PROC_IN_VACUUM | PROC_IN_LOGICAL_DECODING))
 			continue;
 
-		/* shared tables need to take backends in all database into account */
+		/* shared tables need to take backends in all databases into account */
 		h->shared_oldest_nonremovable =
 			TransactionIdOlder(h->shared_oldest_nonremovable, xmin);
 
@@ -1773,11 +1777,26 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 			MyDatabaseId == InvalidOid || proc->databaseId == MyDatabaseId ||
 			proc->databaseId == 0)	/* always include WalSender */
 		{
-			h->data_oldest_nonremovable =
-				TransactionIdOlder(h->data_oldest_nonremovable, xmin);
+			/*
+			 * We can ignore this backend if it's running CREATE INDEX
+			 * CONCURRENTLY or REINDEX CONCURRENTLY on a "safe" index -- but
+			 * only on vacuums of user-defined tables.
+			 */
+			if (!(statusFlags & PROC_IN_SAFE_IC))
+				h->data_oldest_nonremovable =
+					TransactionIdOlder(h->data_oldest_nonremovable, xmin);
+
+			/* Catalog tables need to consider all backends in this db */
+			h->catalog_oldest_nonremovable =
+				TransactionIdOlder(h->catalog_oldest_nonremovable, xmin);
+
 		}
 	}
 
+	/* catalog horizon should never be later than data */
+	Assert(TransactionIdPrecedesOrEquals(h->catalog_oldest_nonremovable,
+										 h->data_oldest_nonremovable));
+
 	/*
 	 * If in recovery fetch oldest xid in KnownAssignedXids, will be applied
 	 * after lock is released.
@@ -1799,6 +1818,8 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 			TransactionIdOlder(h->shared_oldest_nonremovable, kaxmin);
 		h->data_oldest_nonremovable =
 			TransactionIdOlder(h->data_oldest_nonremovable, kaxmin);
+		h->catalog_oldest_nonremovable =
+			TransactionIdOlder(h->catalog_oldest_nonremovable, kaxmin);
 		/* temp relations cannot be accessed in recovery */
 	}
 	else
@@ -1825,6 +1846,9 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 		h->data_oldest_nonremovable =
 			TransactionIdRetreatedBy(h->data_oldest_nonremovable,
 									 vacuum_defer_cleanup_age);
+		h->catalog_oldest_nonremovable =
+			TransactionIdRetreatedBy(h->catalog_oldest_nonremovable,
+									 vacuum_defer_cleanup_age);
 		/* defer doesn't apply to temp relations */
 	}
 
@@ -1847,7 +1871,9 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 	h->shared_oldest_nonremovable =
 		TransactionIdOlder(h->shared_oldest_nonremovable,
 						   h->slot_catalog_xmin);
-	h->catalog_oldest_nonremovable = h->data_oldest_nonremovable;
+	h->catalog_oldest_nonremovable =
+		TransactionIdOlder(h->catalog_oldest_nonremovable,
+						   h->slot_xmin);
 	h->catalog_oldest_nonremovable =
 		TransactionIdOlder(h->catalog_oldest_nonremovable,
 						   h->slot_catalog_xmin);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 260ec7b97e9..d626731723b 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2583,7 +2583,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&vacuum_defer_cleanup_age,
-		0, 0, 1000000,
+		0, 0, 1000000,		/* see ComputeXidHorizons */
 		NULL, NULL, NULL
 	},