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.

src/backend/access/transam/transam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.73 2008/01/01 19:45:48 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.74 2008/03/11 20:20:35 tgl Exp $
  *
  * NOTES
  *	  This file contains the high level access-method interface to the
@@ -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?
  */