Restructure subtransaction handling to reduce resource consumption,

as per recent discussions.  Invent SubTransactionIds that are managed like
CommandIds (ie, counter is reset at start of each top transaction), and
use these instead of TransactionIds to keep track of subtransaction status
in those modules that need it.  This means that a subtransaction does not
need an XID unless it actually inserts/modifies rows in the database.
Accordingly, don't assign it an XID nor take a lock on the XID until it
tries to do that.  This saves a lot of overhead for subtransactions that
are only used for error recovery (eg plpgsql exceptions).  Also, arrange
to release a subtransaction's XID lock as soon as the subtransaction
exits, in both the commit and abort cases.  This avoids holding many
unique locks after a long series of subtransactions.  The price is some
additional overhead in XactLockTableWait, but that seems acceptable.
Finally, restructure the state machine in xact.c to have a more orthogonal
set of states for subtransactions.

1 files changed, 17 insertions, 6 deletions

diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index c30c741f27e..9fa6a1d2406 100644
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2004, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xact.h,v 1.72 2004/09/05 23:01:26 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/xact.h,v 1.73 2004/09/16 16:58:37 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -46,14 +46,21 @@ extern bool XactReadOnly;
  */
 typedef enum
 {
-	XACT_EVENT_ABORT,
 	XACT_EVENT_COMMIT,
-	XACT_EVENT_START_SUB,
-	XACT_EVENT_ABORT_SUB,
-	XACT_EVENT_COMMIT_SUB
+	XACT_EVENT_ABORT
 } XactEvent;
 
-typedef void (*XactCallback) (XactEvent event, TransactionId parentXid, void *arg);
+typedef void (*XactCallback) (XactEvent event, void *arg);
+
+typedef enum
+{
+	SUBXACT_EVENT_START_SUB,
+	SUBXACT_EVENT_COMMIT_SUB,
+	SUBXACT_EVENT_ABORT_SUB
+} SubXactEvent;
+
+typedef void (*SubXactCallback) (SubXactEvent event, SubTransactionId mySubid,
+								 SubTransactionId parentSubid, void *arg);
 
 
 /* ----------------
@@ -101,6 +108,8 @@ extern bool IsTransactionState(void);
 extern bool IsAbortedTransactionBlockState(void);
 extern TransactionId GetTopTransactionId(void);
 extern TransactionId GetCurrentTransactionId(void);
+extern TransactionId GetCurrentTransactionIdIfAny(void);
+extern SubTransactionId GetCurrentSubTransactionId(void);
 extern CommandId GetCurrentCommandId(void);
 extern AbsoluteTime GetCurrentTransactionStartTime(void);
 extern AbsoluteTime GetCurrentTransactionStartTimeUsec(int *usec);
@@ -129,6 +138,8 @@ extern void RequireTransactionChain(void *stmtNode, const char *stmtType);
 extern bool IsInTransactionChain(void *stmtNode);
 extern void RegisterXactCallback(XactCallback callback, void *arg);
 extern void UnregisterXactCallback(XactCallback callback, void *arg);
+extern void RegisterSubXactCallback(SubXactCallback callback, void *arg);
+extern void UnregisterSubXactCallback(SubXactCallback callback, void *arg);
 
 extern void RecordTransactionCommit(void);