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Fix obsolete FSM remarks in nbtree README.

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The free space map has used a dedicated relation fork rather than shared
memory segments for over a decade.
This commit is contained in:
Peter Geoghegan
2021-02-09 11:36:51 -08:00
parent 890d2182a2
commit 31c7fb41e2
1 changed files with 5 additions and 8 deletions
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13
src/backend/access/nbtree/README
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@ -347,14 +347,11 @@ guaranteed to be "visible to everyone". As collateral damage, this
implementation also waits for running XIDs with no snapshots and for
snapshots taken until the next transaction to allocate an XID commits.
Reclaiming a page doesn't actually change its state on disk --- we simply
record it in the shared-memory free space map, from which it will be
handed out the next time a new page is needed for a page split. The
deleted page's contents will be overwritten by the split operation.
(Note: if we find a deleted page with an extremely old transaction
number, it'd be worthwhile to re-mark it with FrozenTransactionId so that
a later xid wraparound can't cause us to think the page is unreclaimable.
But in more normal situations this would be a waste of a disk write.)
Reclaiming a page doesn't actually change the state of the page --- we
simply record it in the free space map, from which it will be handed out
the next time a new page is needed for a page split. The deleted page's
contents will be overwritten by the split operation (it will become the
new right page).
Because we never delete the rightmost page of any level (and in particular
never delete the root), it's impossible for the height of the tree to