Avoid xid error from age() function when run on Hot Standby

diff --git a/src/backend/utils/adt/xid.c b/src/backend/utils/adt/xid.c
index fcde92cb8a8f83958e175f10d223e291b3365a43..7db6bb1a66083e4d4f25e0f6d20f2ed46d1d1721 100644 (file)
--- a/src/backend/utils/adt/xid.c
+++ b/src/backend/utils/adt/xid.c
@@ -93,7 +93,10 @@ Datum
 xid_age(PG_FUNCTION_ARGS)
 {
    TransactionId xid = PG_GETARG_TRANSACTIONID(0);
-   TransactionId now = GetTopTransactionId();
+   TransactionId now = GetTopTransactionIdIfAny();
+
+   if (!TransactionIdIsValid(now))
+       now = ReadNewTransactionId();
 
    /* Permanent XIDs are always infinitely old */
    if (!TransactionIdIsNormal(xid))