Ühel kenal päeval, N, 2007-02-15 kell 10:49, kirjutas Heikki
Linnakangas:
> We already log tuple removals by normal vacuums. We can't use that wal
> entry as it is: if a dead tuple is in the middle of an update chain, it
> needs to be unlinked from the chain. But I don't see any particular
> problem with that, it just needs to be wal logged like every other data
> changing operation.
>
> Do we actually ever want to remove dead tuples from the middle of the
> chain? If a tuple in the middle of the chain is dead, surely every tuple
> before it in the chain is dead as well, and we want to remove them as
> well. I'm thinking, removing tuples from the middle of the chain can be
> problematic, because we'd need to fiddle with the xmin/xmax of the other
> tuples to make them match. Or change the tuple-following logic to not do
> the xmin=xmax check, but it's a nice robustness feature.
What kind of robustness does it provide ? In other words - what failure
scenario does this guard against ?
I can't see the case where the xmin=xmax check can not succeed, at least
not for same page tuples.
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