Make TransactionIdIsInProgress check transam.c's single-item XID status cache

before it goes groveling through the ProcArray.  In situations where the same
recently-committed transaction ID is checked repeatedly by tqual.c, this saves
a lot of shared-memory searches.  And it's cheap enough that it shouldn't
hurt noticeably when it doesn't help.
Concept and patch by Simon, some minor tweaking and comment-cleanup by Tom.

3 files changed, 64 insertions, 29 deletions

diff --git a/src/backend/access/transam/transam.c b/src/backend/access/transam/transam.c
index 1f9eed0a84..c83e4a8237 100644
--- a/src/backend/access/transam/transam.c
+++ b/src/backend/access/transam/transam.c
@@ -25,12 +25,11 @@
 #include "utils/tqual.h"
 
 
-static XidStatus TransactionLogFetch(TransactionId transactionId);
-static void TransactionLogUpdate(TransactionId transactionId,
-					 XidStatus status, XLogRecPtr lsn);
-
 /*
- * Single-item cache for results of TransactionLogFetch.
+ * Single-item cache for results of TransactionLogFetch.  It's worth having
+ * such a cache because we frequently find ourselves repeatedly checking the
+ * same XID, for example when scanning a table just after a bulk insert,
+ * update, or delete.
  */
 static TransactionId cachedFetchXid = InvalidTransactionId;
 static XidStatus cachedFetchXidStatus;
@@ -39,9 +38,14 @@ static XLogRecPtr cachedCommitLSN;
 /* Handy constant for an invalid xlog recptr */
 static const XLogRecPtr InvalidXLogRecPtr = {0, 0};
 
+/* Local functions */
+static XidStatus TransactionLogFetch(TransactionId transactionId);
+static void TransactionLogUpdate(TransactionId transactionId,
+					 XidStatus status, XLogRecPtr lsn);
+
 
 /* ----------------------------------------------------------------
- *		postgres log access method interface
+ *		Postgres log access method interface
  *
  *		TransactionLogFetch
  *		TransactionLogUpdate
@@ -82,8 +86,8 @@ TransactionLogFetch(TransactionId transactionId)
 	xidstatus = TransactionIdGetStatus(transactionId, &xidlsn);
 
 	/*
-	 * DO NOT cache status for unfinished or sub-committed transactions! We
-	 * only cache status that is guaranteed not to change.
+	 * Cache it, but DO NOT cache status for unfinished or sub-committed
+	 * transactions!  We only cache status that is guaranteed not to change.
 	 */
 	if (xidstatus != TRANSACTION_STATUS_IN_PROGRESS &&
 		xidstatus != TRANSACTION_STATUS_SUB_COMMITTED)
@@ -96,12 +100,11 @@ TransactionLogFetch(TransactionId transactionId)
 	return xidstatus;
 }
 
-/* --------------------------------
+/*
  *		TransactionLogUpdate
  *
  * Store the new status of a transaction.  The commit record LSN must be
  * passed when recording an async commit; else it should be InvalidXLogRecPtr.
- * --------------------------------
  */
 static inline void
 TransactionLogUpdate(TransactionId transactionId,
@@ -131,32 +134,27 @@ TransactionLogMultiUpdate(int nxids, TransactionId *xids,
 		TransactionIdSetStatus(xids[i], status, lsn);
 }
 
+
 /* ----------------------------------------------------------------
  *						Interface functions
  *
- *		TransactionId DidCommit
- *		TransactionId DidAbort
- *		TransactionId IsInProgress
+ *		TransactionIdDidCommit
+ *		TransactionIdDidAbort
  *		========
  *		   these functions test the transaction status of
  *		   a specified transaction id.
  *
- *		TransactionId Commit
- *		TransactionId Abort
+ *		TransactionIdCommit
+ *		TransactionIdAbort
  *		========
  *		   these functions set the transaction status
  *		   of the specified xid.
  *
+ * See also TransactionIdIsInProgress, which once was in this module
+ * but now lives in procarray.c.
  * ----------------------------------------------------------------
  */
 
-/* --------------------------------
- *		TransactionId DidCommit
- *		TransactionId DidAbort
- *		TransactionId IsInProgress
- * --------------------------------
- */
-
 /*
  * TransactionIdDidCommit
  *		True iff transaction associated with the identifier did commit.
@@ -262,11 +260,33 @@ TransactionIdDidAbort(TransactionId transactionId)
 	return false;
 }
 
-/* --------------------------------
- *		TransactionId Commit
- *		TransactionId Abort
- * --------------------------------
+/*
+ * TransactionIdIsKnownCompleted
+ *		True iff transaction associated with the identifier is currently
+ *		known to have either committed or aborted.
+ *
+ * This does NOT look into pg_clog but merely probes our local cache
+ * (and so it's not named TransactionIdDidComplete, which would be the
+ * appropriate name for a function that worked that way).  The intended
+ * use is just to short-circuit TransactionIdIsInProgress calls when doing
+ * repeated tqual.c checks for the same XID.  If this isn't extremely fast
+ * then it will be counterproductive.
+ *
+ * Note:
+ *		Assumes transaction identifier is valid.
  */
+bool
+TransactionIdIsKnownCompleted(TransactionId transactionId)
+{
+	if (TransactionIdEquals(transactionId, cachedFetchXid))
+	{
+		/* If it's in the cache at all, it must be completed. */
+		return true;
+	}
+
+	return false;
+}
+
 
 /*
  * TransactionIdCommit
@@ -292,7 +312,6 @@ TransactionIdAsyncCommit(TransactionId transactionId, XLogRecPtr lsn)
 	TransactionLogUpdate(transactionId, TRANSACTION_STATUS_COMMITTED, lsn);
 }
 
-
 /*
  * TransactionIdAbort
  *		Aborts the transaction associated with the identifier.
@@ -352,7 +371,6 @@ TransactionIdAsyncCommitTree(int nxids, TransactionId *xids, XLogRecPtr lsn)
 								  lsn);
 }
 
-
 /*
  * TransactionIdAbortTree
  *		Marks all the given transaction ids as aborted.
@@ -368,6 +386,7 @@ TransactionIdAbortTree(int nxids, TransactionId *xids)
 								  InvalidXLogRecPtr);
 }
 
+
 /*
  * TransactionIdPrecedes --- is id1 logically < id2?
  */
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 69bb1614d4..5927f15a6e 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -60,6 +60,7 @@ static ProcArrayStruct *procArray;
 
 /* counters for XidCache measurement */
 static long xc_by_recent_xmin = 0;
+static long xc_by_known_xact = 0;
 static long xc_by_my_xact = 0;
 static long xc_by_latest_xid = 0;
 static long xc_by_main_xid = 0;
@@ -68,6 +69,7 @@ static long xc_no_overflow = 0;
 static long xc_slow_answer = 0;
 
 #define xc_by_recent_xmin_inc()		(xc_by_recent_xmin++)
+#define xc_by_known_xact_inc()		(xc_by_known_xact++)
 #define xc_by_my_xact_inc()			(xc_by_my_xact++)
 #define xc_by_latest_xid_inc()		(xc_by_latest_xid++)
 #define xc_by_main_xid_inc()		(xc_by_main_xid++)
@@ -79,6 +81,7 @@ static void DisplayXidCache(void);
 #else							/* !XIDCACHE_DEBUG */
 
 #define xc_by_recent_xmin_inc()		((void) 0)
+#define xc_by_known_xact_inc()		((void) 0)
 #define xc_by_my_xact_inc()			((void) 0)
 #define xc_by_latest_xid_inc()		((void) 0)
 #define xc_by_main_xid_inc()		((void) 0)
@@ -354,6 +357,17 @@ TransactionIdIsInProgress(TransactionId xid)
 	}
 
 	/*
+	 * We may have just checked the status of this transaction, so if it is
+	 * already known to be completed, we can fall out without any access to
+	 * shared memory.
+	 */
+	if (TransactionIdIsKnownCompleted(xid))
+	{
+		xc_by_known_xact_inc();
+		return false;
+	}
+
+	/*
 	 * Also, we can handle our own transaction (and subtransactions) without
 	 * any access to shared memory.
 	 */
@@ -1335,8 +1349,9 @@ static void
 DisplayXidCache(void)
 {
 	fprintf(stderr,
-			"XidCache: xmin: %ld, myxact: %ld, latest: %ld, mainxid: %ld, childxid: %ld, nooflo: %ld, slow: %ld\n",
+			"XidCache: xmin: %ld, known: %ld, myxact: %ld, latest: %ld, mainxid: %ld, childxid: %ld, nooflo: %ld, slow: %ld\n",
 			xc_by_recent_xmin,
+			xc_by_known_xact,
 			xc_by_my_xact,
 			xc_by_latest_xid,
 			xc_by_main_xid,
diff --git a/src/include/access/transam.h b/src/include/access/transam.h
index ca64f84319..ba8b253101 100644
--- a/src/include/access/transam.h
+++ b/src/include/access/transam.h
@@ -138,6 +138,7 @@ extern VariableCache ShmemVariableCache;
  */
 extern bool TransactionIdDidCommit(TransactionId transactionId);
 extern bool TransactionIdDidAbort(TransactionId transactionId);
+extern bool TransactionIdIsKnownCompleted(TransactionId transactionId);
 extern void TransactionIdCommit(TransactionId transactionId);
 extern void TransactionIdAsyncCommit(TransactionId transactionId, XLogRecPtr lsn);
 extern void TransactionIdAbort(TransactionId transactionId);