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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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48
src/backend/access/transam/xlog.c
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@ -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 $
*
*-------------------------------------------------------------------------
*/
@ -154,38 +154,16 @@ static TimeLineID recoveryTargetTLI;
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();