Implement lazy XID allocation: transactions that do not modify any database

rows will normally never obtain an XID at all.  We already did things this way
for subtransactions, but this patch extends the concept to top-level
transactions.  In applications where there are lots of short read-only
transactions, this should improve performance noticeably; not so much from
removal of the actual XID-assignments, as from reduction of overhead that's
driven by the rate of XID consumption.  We add a concept of a "virtual
transaction ID" so that active transactions can be uniquely identified even
if they don't have a regular XID.  This is a much lighter-weight concept:
uniqueness of VXIDs is only guaranteed over the short term, and no on-disk
record is made about them.

Florian Pflug, with some editorialization by Tom.

1 files changed, 9 insertions, 39 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 1db33fb26da..5474a91c247 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.279 2007/08/28 23:17:47 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.280 2007/09/05 18:10:47 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -155,37 +155,15 @@ static List *expectedTLIs;
 static TimeLineID curFileTLI;
 
 /*
- * MyLastRecPtr points to the start of the last XLOG record inserted by the
- * current transaction.  If MyLastRecPtr.xrecoff == 0, then the current
- * xact hasn't yet inserted any transaction-controlled XLOG records.
- *
- * Note that XLOG records inserted outside transaction control are not
- * reflected into MyLastRecPtr.  They do, however, cause MyXactMadeXLogEntry
- * to be set true.	The latter can be used to test whether the current xact
- * made any loggable changes (including out-of-xact changes, such as
- * sequence updates).
- *
- * When we insert/update/delete a tuple in a temporary relation, we do not
- * make any XLOG record, since we don't care about recovering the state of
- * the temp rel after a crash.	However, we will still need to remember
- * whether our transaction committed or aborted in that case.  So, we must
- * set MyXactMadeTempRelUpdate true to indicate that the XID will be of
- * interest later.
- */
-XLogRecPtr	MyLastRecPtr = {0, 0};
-
-bool		MyXactMadeXLogEntry = false;
-
-bool		MyXactMadeTempRelUpdate = false;
-
-/*
  * ProcLastRecPtr points to the start of the last XLOG record inserted by the
- * current backend.  It is updated for all inserts, transaction-controlled
- * or not.	ProcLastRecEnd is similar but points to end+1 of last record.
+ * current backend.  It is updated for all inserts.  XactLastRecEnd points to
+ * end+1 of the last record, and is reset when we end a top-level transaction,
+ * or start a new one; so it can be used to tell if the current transaction has
+ * created any XLOG records.
  */
 static XLogRecPtr ProcLastRecPtr = {0, 0};
 
-XLogRecPtr	ProcLastRecEnd = {0, 0};
+XLogRecPtr	XactLastRecEnd = {0, 0};
 
 /*
  * RedoRecPtr is this backend's local copy of the REDO record pointer
@@ -488,15 +466,10 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
 	bool		updrqst;
 	bool		doPageWrites;
 	bool		isLogSwitch = (rmid == RM_XLOG_ID && info == XLOG_SWITCH);
-	bool		no_tran = (rmid == RM_XLOG_ID);
 
+	/* info's high bits are reserved for use by me */
 	if (info & XLR_INFO_MASK)
-	{
-		if ((info & XLR_INFO_MASK) != XLOG_NO_TRAN)
-			elog(PANIC, "invalid xlog info mask %02X", (info & XLR_INFO_MASK));
-		no_tran = true;
-		info &= ~XLR_INFO_MASK;
-	}
+		elog(PANIC, "invalid xlog info mask %02X", info);
 
 	/*
 	 * In bootstrap mode, we don't actually log anything but XLOG resources;
@@ -856,11 +829,8 @@ begin:;
 #endif
 
 	/* Record begin of record in appropriate places */
-	if (!no_tran)
-		MyLastRecPtr = RecPtr;
 	ProcLastRecPtr = RecPtr;
 	Insert->PrevRecord = RecPtr;
-	MyXactMadeXLogEntry = true;
 
 	Insert->currpos += SizeOfXLogRecord;
 	freespace -= SizeOfXLogRecord;
@@ -1018,7 +988,7 @@ begin:;
 		SpinLockRelease(&xlogctl->info_lck);
 	}
 
-	ProcLastRecEnd = RecPtr;
+	XactLastRecEnd = RecPtr;
 
 	END_CRIT_SECTION();