HOT updates. When we update a tuple without changing any of its indexed

columns, and the new version can be stored on the same heap page, we no longer
generate extra index entries for the new version.  Instead, index searches
follow the HOT-chain links to ensure they find the correct tuple version.

In addition, this patch introduces the ability to "prune" dead tuples on a
per-page basis, without having to do a complete VACUUM pass to recover space.
VACUUM is still needed to clean up dead index entries, however.

Pavan Deolasee, with help from a bunch of other people.

src/backend/storage/buffer/bufmgr.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.223 2007/06/30 19:12:01 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.224 2007/09/20 17:56:31 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2066,6 +2066,55 @@ LockBufferForCleanup(Buffer buffer)
 	}
 }
 
+/*
+ * ConditionalLockBufferForCleanup - as above, but don't wait to get the lock
+ *
+ * We won't loop, but just check once to see if the pin count is OK.  If
+ * not, return FALSE with no lock held.
+ */ 
+bool
+ConditionalLockBufferForCleanup(Buffer buffer)
+{
+	volatile BufferDesc *bufHdr;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		Assert(LocalRefCount[-buffer - 1] > 0);
+		if (LocalRefCount[-buffer - 1] != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	Assert(PrivateRefCount[buffer - 1] > 0);
+	if (PrivateRefCount[buffer - 1] != 1)
+		return false;
+
+	/* Try to acquire lock */
+	if (!ConditionalLockBuffer(buffer))
+		return false;
+
+	bufHdr = &BufferDescriptors[buffer - 1];
+	LockBufHdr(bufHdr);
+	Assert(bufHdr->refcount > 0);
+	if (bufHdr->refcount == 1)
+	{
+		/* Successfully acquired exclusive lock with pincount 1 */
+		UnlockBufHdr(bufHdr);
+		return true;
+	}
+
+	/* Failed, so release the lock */
+	UnlockBufHdr(bufHdr);
+	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+	return false;
+}
+
+
 /*
  *	Functions for buffer I/O handling
  *