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, 3 insertions, 10 deletions

diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 9665d129541..419c8656065 100644
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -26,7 +26,7 @@
  * 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/clog.c,v 1.43 2007/08/01 22:45:07 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/clog.c,v 1.44 2007/09/05 18:10:47 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -423,10 +423,6 @@ CLOGPagePrecedes(int page1, int page2)
 
 /*
  * Write a ZEROPAGE xlog record
- *
- * Note: xlog record is marked as outside transaction control, since we
- * want it to be redone whether the invoking transaction commits or not.
- * (Besides which, this is normally done just before entering a transaction.)
  */
 static void
 WriteZeroPageXlogRec(int pageno)
@@ -437,7 +433,7 @@ WriteZeroPageXlogRec(int pageno)
 	rdata.len = sizeof(int);
 	rdata.buffer = InvalidBuffer;
 	rdata.next = NULL;
-	(void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE | XLOG_NO_TRAN, &rdata);
+	(void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE, &rdata);
 }
 
 /*
@@ -445,9 +441,6 @@ WriteZeroPageXlogRec(int pageno)
  *
  * We must flush the xlog record to disk before returning --- see notes
  * in TruncateCLOG().
- *
- * Note: xlog record is marked as outside transaction control, since we
- * want it to be redone whether the invoking transaction commits or not.
  */
 static void
 WriteTruncateXlogRec(int pageno)
@@ -459,7 +452,7 @@ WriteTruncateXlogRec(int pageno)
 	rdata.len = sizeof(int);
 	rdata.buffer = InvalidBuffer;
 	rdata.next = NULL;
-	recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE | XLOG_NO_TRAN, &rdata);
+	recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE, &rdata);
 	XLogFlush(recptr);
 }