Assign coordinator local timestamp when transaction timestamp is requested

before or without assigning XID to the transaction.

GTM supplied timestamp is available only when a transaction has started on the
GTM. But for read-only transactions or transactions which are yet to do any
database write activity, we avoid going to GTM for performance reason. For such
cases, use coordinator local timestamp and continue to use the same for rest of
the transaction.

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index e615c7e2c94fea9ba768d11270e15a2f455b78d7..f42d8d3d1cb6cd13e26b7436a93608108badaffd 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -685,7 +685,8 @@ AssignTransactionId(TransactionState s)
                s->transactionId = GetNewTransactionId(isSubXact, &received_tp, &gtm_timestamp);
                if (received_tp)
                {
-                       GTMxactStartTimestamp = (TimestampTz) gtm_timestamp;
+                       if (GTMxactStartTimestamp == 0)
+                               GTMxactStartTimestamp = (TimestampTz) gtm_timestamp;
                        GTMdeltaTimestamp = GTMxactStartTimestamp - stmtStartTimestamp;
                }
        }
@@ -905,6 +906,8 @@ GetCurrentTransactionStartTimestamp(void)
         * from GTM along with GXID.
         */
 #ifdef PGXC
+       if (GTMxactStartTimestamp == 0)
+               GTMxactStartTimestamp = xactStartTimestamp;
        return GTMxactStartTimestamp;
 #else
        return xactStartTimestamp;
@@ -964,6 +967,8 @@ GetCurrentTransactionStopTimestamp(void)
 TimestampTz
 GetCurrentGTMStartTimestamp(void)
 {
+       if (GTMxactStartTimestamp == 0)
+               GTMxactStartTimestamp = xactStartTimestamp;
        return GTMxactStartTimestamp;
 }
 #endif
@@ -995,7 +1000,8 @@ SetCurrentTransactionStopTimestamp(void)
 void
 SetCurrentGTMDeltaTimestamp(TimestampTz timestamp)
 {
-       GTMxactStartTimestamp = timestamp;
+       if (GTMxactStartTimestamp == 0)
+               GTMxactStartTimestamp = timestamp;
        GTMdeltaTimestamp = GTMxactStartTimestamp - xactStartTimestamp;
 }
 #endif
@@ -2193,7 +2199,9 @@ StartTransaction(void)
        xactStopTimestamp = 0;
 #ifdef PGXC
        /* For Postgres-XC, transaction start timestamp has to follow the GTM timeline */
-       pgstat_report_xact_timestamp(GTMxactStartTimestamp);
+       pgstat_report_xact_timestamp(GTMxactStartTimestamp ?
+                       GTMxactStartTimestamp :
+                       xactStartTimestamp);
 #else
        pgstat_report_xact_timestamp(xactStartTimestamp);
 #endif
@@ -2650,6 +2658,7 @@ CommitTransaction(void)
 
 #ifdef PGXC
        AtEOXact_Remote();
+       GTMxactStartTimestamp = 0;
 #endif
 }
 
@@ -3026,6 +3035,7 @@ PrepareTransaction(void)
        CleanGTMCallbacks();
 #ifdef XCP     
        AtEOXact_Remote();      
+       GTMxactStartTimestamp = 0;
 #endif 
 #endif
 
@@ -3318,6 +3328,7 @@ AbortTransaction(void)
 
 #ifdef PGXC
        AtEOXact_Remote();
+       GTMxactStartTimestamp = 0;
 #endif
 
 }