Will an INSERT reuse rows no longer visible caused by an UPDATE or DELETE, or if an UPDATE will reuse rows expired by an invisible DELETE? It seems both of these would be possible and useful.
As the patch stands today, we don't take any pains to update the FSM information after pruning and defraging the page. So FSM would not consider the page for either INSERT or COLD UPDATE. Of course, the page can still be used for INSERT if relation->rd_targetblock is somehow set to this page.
OTOH UPDATE will always reuse the dead space of either expired updated rows or deleted rows or even aborted rows.
