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

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

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/*
 * 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)));
}

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Shouldn't this be HeapTupleHeaderData and not HeapTupleData ?

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

Sailesh Krishnamurthy