Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

By the way, I still wonder if there's any way for a new tuple to get
inserted in the place where a HOT redirect would be pointing to, and
have it be marked as Frozen, where the old redirect contains a
non-invalid Xmax. I tried to think of a way for that to happen, but
couldn't think of anything.

What I imagine is a sequence like this:

1. insert a tuple
2. HOT-update a tuple
3. prune the page, making lp 1 be a redirect (lp 2 is the new tuple)
4. start transaction
5. HOT-update the tuple again, creating HOT in lp 3
6. abort transaction (leaving aborted update in lp 3)
7. somehow remove tuple from lp 3, make slot available
8. new transaction comes along, inserts tuple in lp 3
9. somebody freezes tuple in lp3 (???)

Then we follow the HOT chain, see that Xmin=2 in lp3 and conclude that
the tuple is part of the chain because of an xid "match".

Basically from step 7 onwards I don't think this is possible, but maybe
I'm just blind.

Maybe we can forestall the problem by checking whether the Xmax
TransactionIdIsCurrentTransaction || TransactionIdDidCommit (or some
variant thereof). This would be very slow but safer; and in 9.4 and up
we'd only need to do it if the xmin value is actually FrozenXid which
should be rare (only in pages upgraded from 9.3).

--
Álvaro Herrera https://www.2ndQuadrant.com/
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