From 5829387381d2e4edf84652bb5a712f6185860670 Mon Sep 17 00:00:00 2001
From: Simon Riggs <simon@2ndQuadrant.com>
Date: Wed, 9 May 2012 13:56:24 +0100
Subject: [PATCH] Avoid xid error from age() function when run on Hot Standby

---
 src/backend/utils/adt/xid.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/backend/utils/adt/xid.c b/src/backend/utils/adt/xid.c
index 1829b8242d..eeda081ad1 100644
--- 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))
-- 
2.40.0