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, 13 insertions, 12 deletions

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index fb2a82134e0..6ebdaa6f567 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/file/fd.c,v 1.112 2004/08/29 05:06:47 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/file/fd.c,v 1.113 2004/09/16 16:58:32 tgl Exp $
  *
  * NOTES:
  *
@@ -123,7 +123,7 @@ typedef struct vfd
 {
 	signed short fd;			/* current FD, or VFD_CLOSED if none */
 	unsigned short fdstate;		/* bitflags for VFD's state */
-	TransactionId create_xid;	/* for XACT_TEMPORARY fds, creating Xid */
+	SubTransactionId create_subid;	/* for TEMPORARY fds, creating subxact */
 	File		nextFree;		/* link to next free VFD, if in freelist */
 	File		lruMoreRecently;	/* doubly linked recency-of-use list */
 	File		lruLessRecently;
@@ -171,7 +171,7 @@ typedef struct
 		FILE	   *file;
 		DIR		   *dir;
 	}			desc;
-	TransactionId create_xid;
+	SubTransactionId create_subid;
 } AllocateDesc;
 
 static int	numAllocatedDescs = 0;
@@ -887,7 +887,7 @@ OpenTemporaryFile(bool interXact)
 	if (!interXact)
 	{
 		VfdCache[file].fdstate |= FD_XACT_TEMPORARY;
-		VfdCache[file].create_xid = GetCurrentTransactionId();
+		VfdCache[file].create_subid = GetCurrentSubTransactionId();
 	}
 
 	return file;
@@ -1166,7 +1166,7 @@ TryAgain:
 
 		desc->kind = AllocateDescFile;
 		desc->desc.file = file;
-		desc->create_xid = GetCurrentTransactionId();
+		desc->create_subid = GetCurrentSubTransactionId();
 		numAllocatedDescs++;
 		return desc->desc.file;
 	}
@@ -1281,7 +1281,7 @@ TryAgain:
 
 		desc->kind = AllocateDescDir;
 		desc->desc.dir = dir;
-		desc->create_xid = GetCurrentTransactionId();
+		desc->create_subid = GetCurrentSubTransactionId();
 		numAllocatedDescs++;
 		return desc->desc.dir;
 	}
@@ -1359,10 +1359,11 @@ closeAllVfds(void)
  *
  * Take care of subtransaction commit/abort.  At abort, we close temp files
  * that the subtransaction may have opened.  At commit, we reassign the
- * files that were opened to the parent transaction.
+ * files that were opened to the parent subtransaction.
  */
 void
-AtEOSubXact_Files(bool isCommit, TransactionId myXid, TransactionId parentXid)
+AtEOSubXact_Files(bool isCommit, SubTransactionId mySubid,
+				  SubTransactionId parentSubid)
 {
 	Index		i;
 
@@ -1374,10 +1375,10 @@ AtEOSubXact_Files(bool isCommit, TransactionId myXid, TransactionId parentXid)
 			unsigned short fdstate = VfdCache[i].fdstate;
 
 			if ((fdstate & FD_XACT_TEMPORARY) &&
-				VfdCache[i].create_xid == myXid)
+				VfdCache[i].create_subid == mySubid)
 			{
 				if (isCommit)
-					VfdCache[i].create_xid = parentXid;
+					VfdCache[i].create_subid = parentSubid;
 				else if (VfdCache[i].fileName != NULL)
 					FileClose(i);
 			}
@@ -1386,10 +1387,10 @@ AtEOSubXact_Files(bool isCommit, TransactionId myXid, TransactionId parentXid)
 
 	for (i = 0; i < numAllocatedDescs; i++)
 	{
-		if (allocatedDescs[i].create_xid == myXid)
+		if (allocatedDescs[i].create_subid == mySubid)
 		{
 			if (isCommit)
-				allocatedDescs[i].create_xid = parentXid;
+				allocatedDescs[i].create_subid = parentSubid;
 			else
 			{
 				/* have to recheck the item after FreeDesc (ugly) */