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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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21
src/backend/commands/sequence.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/commands/sequence.c,v 1.143 2007/02/01 19:10:26 momjian Exp $
* $PostgreSQL: pgsql/src/backend/commands/sequence.c,v 1.144 2007/09/05 18:10:47 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -25,6 +25,7 @@
#include "commands/tablecmds.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "storage/proc.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/lsyscache.h"
@ -63,7 +64,7 @@ typedef struct SeqTableData
{
struct SeqTableData *next; /* link to next SeqTable object */
Oid relid; /* pg_class OID of this sequence */
TransactionId xid; /* xact in which we last did a seq op */
LocalTransactionId lxid; /* xact in which we last did a seq op */
int64 last; /* value last returned by nextval */
int64 cached; /* last value already cached for nextval */
/* if last != cached, we have not used up all the cached values */
@ -282,7 +283,7 @@ DefineSequence(CreateSeqStmt *seq)
rdata[1].buffer = InvalidBuffer;
rdata[1].next = NULL;
recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG | XLOG_NO_TRAN, rdata);
recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG, rdata);
PageSetLSN(page, recptr);
PageSetTLI(page, ThisTimeLineID);
@ -366,7 +367,7 @@ AlterSequence(AlterSeqStmt *stmt)
rdata[1].buffer = InvalidBuffer;
rdata[1].next = NULL;
recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG | XLOG_NO_TRAN, rdata);
recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG, rdata);
PageSetLSN(page, recptr);
PageSetTLI(page, ThisTimeLineID);
@ -594,7 +595,7 @@ nextval_internal(Oid relid)
rdata[1].buffer = InvalidBuffer;
rdata[1].next = NULL;
recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG | XLOG_NO_TRAN, rdata);
recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG, rdata);
PageSetLSN(page, recptr);
PageSetTLI(page, ThisTimeLineID);
@ -764,7 +765,7 @@ do_setval(Oid relid, int64 next, bool iscalled)
rdata[1].buffer = InvalidBuffer;
rdata[1].next = NULL;
recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG | XLOG_NO_TRAN, rdata);
recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG, rdata);
PageSetLSN(page, recptr);
PageSetTLI(page, ThisTimeLineID);
@ -825,10 +826,10 @@ setval3_oid(PG_FUNCTION_ARGS)
static Relation
open_share_lock(SeqTable seq)
{
TransactionId thisxid = GetTopTransactionId();
LocalTransactionId thislxid = MyProc->lxid;
/* Get the lock if not already held in this xact */
if (seq->xid != thisxid)
if (seq->lxid != thislxid)
{
ResourceOwner currentOwner;
@ -848,7 +849,7 @@ open_share_lock(SeqTable seq)
CurrentResourceOwner = currentOwner;
/* Flag that we have a lock in the current xact */
seq->xid = thisxid;
seq->lxid = thislxid;
}
/* We now know we have AccessShareLock, and can safely open the rel */
@ -891,7 +892,7 @@ init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel)
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory")));
elm->relid = relid;
elm->xid = InvalidTransactionId;
elm->lxid = InvalidLocalTransactionId;
/* increment is set to 0 until we do read_info (see currval) */
elm->last = elm->cached = elm->increment = 0;
elm->next = seqtab;