if (VM_ALL_VISIBLE(rel,blkno,&vmbuffer))

{

The values stored in the free space map are not exact. They're rounded

to precision of 1/256th of <symbol>BLCKSZ</symbol> (32 bytes with default <symbol>BLCKSZ</symbol>), and

they're not kept fully up-to-date as tuples are inserted and updated.

In addition, small tables don't have a free space map, so these functions

will return zero even if free space is available.

</para>

<para>

bit set, then it is assumed to contain no dead tuples). For such

pages, it derives the free space value from the free space map, and

assumes that the rest of the space on the page is taken up by live

tuples.

tuples. Small tables don't have a free space map, so in that case

this function will report zero free space, likewise inflating the

approximate tuple length.

</para>

<para>