Introduce the concept of relation forks. An smgr relation can now consist

of multiple forks, and each fork can be created and grown separately.

The bulk of this patch is about changing the smgr API to include an extra
ForkNumber argument in every smgr function. Also, smgrscheduleunlink and
smgrdounlink no longer implicitly call smgrclose, because other forks might
still exist after unlinking one. The callers of those functions have been
modified to call smgrclose instead.

This patch in itself doesn't have any user-visible effect, but provides the
infrastructure needed for upcoming patches. The additional forks envisioned
are a rewritten FSM implementation that doesn't rely on a fixed-size shared
memory block, and a visibility map to allow skipping portions of a table in
VACUUM that have no dead tuples.

src/include/access/heapam.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/heapam.h,v 1.137 2008/06/19 00:46:06 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/access/heapam.h,v 1.138 2008/08/11 11:05:11 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -127,7 +127,8 @@ extern XLogRecPtr log_heap_clean(Relation reln, Buffer buffer,
 extern XLogRecPtr log_heap_freeze(Relation reln, Buffer buffer,
 				TransactionId cutoff_xid,
 				OffsetNumber *offsets, int offcnt);
-extern XLogRecPtr log_newpage(RelFileNode *rnode, BlockNumber blk, Page page);
+extern XLogRecPtr log_newpage(RelFileNode *rnode, ForkNumber forkNum,
+							  BlockNumber blk, Page page);
 
 /* in heap/pruneheap.c */
 extern void heap_page_prune_opt(Relation relation, Buffer buffer,

src/include/access/htup.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.100 2008/07/13 20:45:47 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.101 2008/08/11 11:05:11 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -670,6 +670,7 @@ typedef struct xl_heap_clean
 typedef struct xl_heap_newpage
 {
 	RelFileNode node;
+	ForkNumber	forknum;
 	BlockNumber blkno;			/* location of new page */
 	/* entire page contents follow at end of record */
 } xl_heap_newpage;

src/include/access/xact.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xact.h,v 1.94 2008/03/04 19:54:06 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/xact.h,v 1.95 2008/08/11 11:05:11 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -88,10 +88,10 @@ typedef void (*SubXactCallback) (SubXactEvent event, SubTransactionId mySubid,
 typedef struct xl_xact_commit
 {
 	TimestampTz xact_time;		/* time of commit */
-	int			nrels;			/* number of RelFileNodes */
+	int			nrels;			/* number of RelFileForks */
 	int			nsubxacts;		/* number of subtransaction XIDs */
-	/* Array of RelFileNode(s) to drop at commit */
-	RelFileNode xnodes[1];		/* VARIABLE LENGTH ARRAY */
+	/* Array of RelFileFork(s) to drop at commit */
+	RelFileFork	xnodes[1];		/* VARIABLE LENGTH ARRAY */
 	/* ARRAY OF COMMITTED SUBTRANSACTION XIDs FOLLOWS */
 } xl_xact_commit;
 
@@ -100,10 +100,10 @@ typedef struct xl_xact_commit
 typedef struct xl_xact_abort
 {
 	TimestampTz xact_time;		/* time of abort */
-	int			nrels;			/* number of RelFileNodes */
+	int			nrels;			/* number of RelFileForks */
 	int			nsubxacts;		/* number of subtransaction XIDs */
-	/* Array of RelFileNode(s) to drop at abort */
-	RelFileNode xnodes[1];		/* VARIABLE LENGTH ARRAY */
+	/* Array of RelFileFork(s) to drop at abort */
+	RelFileFork	xnodes[1];		/* VARIABLE LENGTH ARRAY */
 	/* ARRAY OF ABORTED SUBTRANSACTION XIDs FOLLOWS */
 } xl_xact_abort;
 

src/include/access/xlog_internal.h

@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xlog_internal.h,v 1.23 2008/02/17 02:09:30 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/xlog_internal.h,v 1.24 2008/08/11 11:05:11 heikki Exp $
  */
 #ifndef XLOG_INTERNAL_H
 #define XLOG_INTERNAL_H
@@ -40,6 +40,7 @@
 typedef struct BkpBlock
 {
 	RelFileNode node;			/* relation containing block */
+	ForkNumber	fork;			/* fork within the relation */
 	BlockNumber block;			/* block number */
 	uint16		hole_offset;	/* number of bytes before "hole" */
 	uint16		hole_length;	/* number of bytes in "hole" */

src/include/access/xlogutils.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xlogutils.h,v 1.25 2008/06/19 00:46:06 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/access/xlogutils.h,v 1.26 2008/08/11 11:05:11 heikki Exp $
  */
 #ifndef XLOG_UTILS_H
 #define XLOG_UTILS_H
@@ -19,11 +19,14 @@
 
 extern void XLogCheckInvalidPages(void);
 
-extern void XLogDropRelation(RelFileNode rnode);
+extern void XLogDropRelation(RelFileNode rnode, ForkNumber forknum);
 extern void XLogDropDatabase(Oid dbid);
-extern void XLogTruncateRelation(RelFileNode rnode, BlockNumber nblocks);
+extern void XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
+								 BlockNumber nblocks);
 
 extern Buffer XLogReadBuffer(RelFileNode rnode, BlockNumber blkno, bool init);
+extern Buffer XLogReadBufferWithFork(RelFileNode rnode, ForkNumber forknum,
+									 BlockNumber blkno, bool init);
 
 extern Relation CreateFakeRelcacheEntry(RelFileNode rnode);
 extern void FreeFakeRelcacheEntry(Relation fakerel);