Here is the definition of relation_byte_size() in optimizer/path/costsize.c:

----------------------------------------------------------------------
/*
 * relation_byte_size
 *        Estimate the storage space in bytes for a given number of tuples
 *        of a given width (size in bytes).
 */
static double
relation_byte_size(double tuples, int width)
{
        return tuples * (MAXALIGN(width) + MAXALIGN(sizeof(HeapTupleData)));
}

----------------------------------------------------------------------

Shouldn't this be HeapTupleHeaderData and not HeapTupleData ?

(Of course, from a costing perspective these shouldn't be very different but ...)

Sailesh Krishnamurthy

src/backend/optimizer/path/costsize.c

@@ -49,7 +49,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.117 2003/12/03 17:45:07 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.118 2003/12/18 03:46:45 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1969,7 +1969,7 @@ set_rel_width(Query *root, RelOptInfo *rel)
 static double
 relation_byte_size(double tuples, int width)
 {
-	return tuples * (MAXALIGN(width) + MAXALIGN(sizeof(HeapTupleData)));
+	return tuples * (MAXALIGN(width) + MAXALIGN(sizeof(HeapTupleHeaderData)));
 }
 
 /*