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

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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.
This commit is contained in:
Tom Lane
2007-09-05 18:10:48 +00:00
parent 2e74c53ec1
commit 295e63983d
34 changed files with 1002 additions and 754 deletions
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13
src/backend/access/transam/clog.c
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@@ -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);
}