From b06679e01244d33304b71a6a44c7cc86173617b3 Mon Sep 17 00:00:00 2001
From: Simon Riggs <simon@2ndQuadrant.com>
Date: Fri, 11 May 2012 14:36:24 +0100
Subject: [PATCH] Ensure age() returns a stable value rather than the latest
value
---
src/backend/access/transam/xact.c | 22 ++++++++++++++++++++++
src/backend/utils/adt/xid.c | 8 +++-----
src/include/access/xact.h | 1 +
3 files changed, 26 insertions(+), 5 deletions(-)
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 7f412b10d75..1654a0e5c73 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -391,6 +391,28 @@ GetCurrentTransactionIdIfAny(void)
}
+/*
+ * GetStableLatestTransactionIdIfAny
+ *
+ * Get the latest XID once and then return same value for rest of transaction.
+ * Acts as a useful reference point for maintenance tasks.
+ */
+TransactionId
+GetStableLatestTransactionId(void)
+{
+ static LocalTransactionId lxid = InvalidLocalTransactionId;
+ static TransactionId stablexid = InvalidTransactionId;
+
+ if (lxid != MyProc->lxid ||
+ !TransactionIdIsValid(stablexid))
+ {
+ lxid = MyProc->lxid;
+ stablexid = ReadNewTransactionId();
+ }
+
+ return stablexid;
+}
+
/*
* AssignTransactionId
*
diff --git a/src/backend/utils/adt/xid.c b/src/backend/utils/adt/xid.c
index eeda081ad18..da9da8a29b9 100644
--- a/src/backend/utils/adt/xid.c
+++ b/src/backend/utils/adt/xid.c
@@ -19,6 +19,7 @@
#include "access/transam.h"
#include "access/xact.h"
#include "libpq/pqformat.h"
+#include "storage/proc.h"
#include "utils/builtins.h"
#define PG_GETARG_TRANSACTIONID(n) DatumGetTransactionId(PG_GETARG_DATUM(n))
@@ -87,16 +88,13 @@ xideq(PG_FUNCTION_ARGS)
}
/*
- * xid_age - compute age of an XID (relative to current xact)
+ * xid_age - compute age of an XID (relative to latest stable xid)
*/
Datum
xid_age(PG_FUNCTION_ARGS)
{
TransactionId xid = PG_GETARG_TRANSACTIONID(0);
- TransactionId now = GetTopTransactionIdIfAny();
-
- if (!TransactionIdIsValid(now))
- now = ReadNewTransactionId();
+ TransactionId now = GetStableLatestTransactionId();
/* Permanent XIDs are always infinitely old */
if (!TransactionIdIsNormal(xid))
diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index 20e344e5b73..50f181307ff 100644
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -209,6 +209,7 @@ extern TransactionId GetTopTransactionId(void);
extern TransactionId GetTopTransactionIdIfAny(void);
extern TransactionId GetCurrentTransactionId(void);
extern TransactionId GetCurrentTransactionIdIfAny(void);
+extern TransactionId GetStableLatestTransactionId(void);
extern SubTransactionId GetCurrentSubTransactionId(void);
extern CommandId GetCurrentCommandId(bool used);
extern TimestampTz GetCurrentTransactionStartTimestamp(void);
--
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