File Manager (cont)
Free space map (Oid_fsm):
- indicates where free space is in data pages
- "free" space is only free after
VACUUM
(DELETE simply marks tuples as no longer in use xmax)
Visibility map (Oid_vm):
- indicates pages where all tuples are "visible"
(visible = accessible to all currently active transactions)
- such pages can be ignored by
VACUUM
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