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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/include/access/xlog.h
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@@ -6,7 +6,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.82 2007/08/01 22:45:09 tgl Exp $
* $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.83 2007/09/05 18:10:48 tgl Exp $
*/
#ifndef XLOG_H
#define XLOG_H
@@ -85,12 +85,6 @@ typedef struct XLogRecord
*/
#define XLR_BKP_REMOVABLE 0x01
/*
* Sometimes we log records which are out of transaction control.
* Rmgr may "or" XLOG_NO_TRAN into info passed to XLogInsert to indicate this.
*/
#define XLOG_NO_TRAN XLR_INFO_MASK
/* Sync methods */
#define SYNC_METHOD_FSYNC 0
#define SYNC_METHOD_FDATASYNC 1
@@ -139,10 +133,7 @@ typedef struct XLogRecData
extern TimeLineID ThisTimeLineID; /* current TLI */
extern bool InRecovery;
extern XLogRecPtr MyLastRecPtr;
extern bool MyXactMadeXLogEntry;
extern bool MyXactMadeTempRelUpdate;
extern XLogRecPtr ProcLastRecEnd;
extern XLogRecPtr XactLastRecEnd;
/* these variables are GUC parameters related to XLOG */
extern int CheckPointSegments;