Tom Lane wrote:
I wrote:
I've been analyzing Ed L's recent report of index corruption:
http://archives.postgresql.org/pgsql-general/2006-10/msg01183.php
Auch. That's nasty indeed.
So I think the rule needs to be "don't delete the rightmost child unless
it's the only child, in which case you can delete the parent too --- but
the same restriction must be observed at the next level up".
....
The concept of a half-dead page would remain, but it'd be a transient
state that would normally only persist for a moment between atomic
page-delete actions. If we crash between two such actions, the
half-dead page would remain present, but would be found and cleaned up
by the next VACUUM. In the meantime it wouldn't cause any problem
because the keyspace it gives up will belong to a sibling of the same
parent at whatever level the delete is ultimately supposed to stop at,
and so inserts and even splits in that keyspace won't create an
inconsistency.
I don't understand how this "in the meantime" thing works. I tried to
work out a step-by-step example, could you take a look at it? See
http://users.tkk.fi/~hlinnaka/pgsql/btree-deletion-bug/
...
Comments? Have I missed anything?
It took me a lot of time with pen and paper to understand the issue. And
I'm not sure I still understood it fully. The logic is very complex,
which is bad for maintainability in itself :(.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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