diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 9759ae11a97..c9e0bf4bbfe 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -1066,6 +1066,12 @@ vac_truncate_clog(TransactionId frozenXID,
 	/*
 	 * Scan pg_database to compute the minimum datfrozenxid/datminmxid
 	 *
+	 * Since vac_update_datfrozenxid updates datfrozenxid/datminmxid in-place,
+	 * the values could change while we look at them.  Fetch each one just
+	 * once to ensure sane behavior of the comparison logic.  (Here, as in
+	 * many other places, we assume that fetching or updating an XID in shared
+	 * storage is atomic.)
+	 *
 	 * Note: we need not worry about a race condition with new entries being
 	 * inserted by CREATE DATABASE.  Any such entry will have a copy of some
 	 * existing DB's datfrozenxid, and that source DB cannot be ours because
@@ -1081,10 +1087,12 @@ vac_truncate_clog(TransactionId frozenXID,
 
 	while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
 	{
-		Form_pg_database dbform = (Form_pg_database) GETSTRUCT(tuple);
+		volatile FormData_pg_database *dbform = (Form_pg_database) GETSTRUCT(tuple);
+		TransactionId datfrozenxid = dbform->datfrozenxid;
+		TransactionId datminmxid = dbform->datminmxid;
 
-		Assert(TransactionIdIsNormal(dbform->datfrozenxid));
-		Assert(MultiXactIdIsValid(dbform->datminmxid));
+		Assert(TransactionIdIsNormal(datfrozenxid));
+		Assert(MultiXactIdIsValid(datminmxid));
 
 		/*
 		 * If things are working properly, no database should have a
@@ -1095,21 +1103,21 @@ vac_truncate_clog(TransactionId frozenXID,
 		 * databases have been scanned and cleaned up.  (We will issue the
 		 * "already wrapped" warning if appropriate, though.)
 		 */
-		if (TransactionIdPrecedes(lastSaneFrozenXid, dbform->datfrozenxid) ||
-			MultiXactIdPrecedes(lastSaneMinMulti, dbform->datminmxid))
+		if (TransactionIdPrecedes(lastSaneFrozenXid, datfrozenxid) ||
+			MultiXactIdPrecedes(lastSaneMinMulti, datminmxid))
 			bogus = true;
 
-		if (TransactionIdPrecedes(nextXID, dbform->datfrozenxid))
+		if (TransactionIdPrecedes(nextXID, datfrozenxid))
 			frozenAlreadyWrapped = true;
-		else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
+		else if (TransactionIdPrecedes(datfrozenxid, frozenXID))
 		{
-			frozenXID = dbform->datfrozenxid;
+			frozenXID = datfrozenxid;
 			oldestxid_datoid = HeapTupleGetOid(tuple);
 		}
 
-		if (MultiXactIdPrecedes(dbform->datminmxid, minMulti))
+		if (MultiXactIdPrecedes(datminmxid, minMulti))
 		{
-			minMulti = dbform->datminmxid;
+			minMulti = datminmxid;
 			minmulti_datoid = HeapTupleGetOid(tuple);
 		}
 	}