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, 11 insertions, 10 deletions

diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index bd06bfb5da3..cb2a1380caf 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -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;