Re: checkpoint clarifications needed

Tom DalPozzo wrote:

> Hi,
> so let's suppose that the WAL is:
> LSN 10: start transaction 123
> LSN 11: update tuple 100
> checkpoint position here (not a record but just for understanding)
> LSN 12: update tuple 100
> LSN 13: update tuple 100
> LSN 14: checkpoint record ( postion=11)
> LSN 15: update tuple 100
> and that the system crashes now, before ending to write all the
> transaction's recs to the WAL (other updates and commit record missing).
>
> At the replay, starting from LSN 12, the entire page we had at LSN 11 is
> written to the disk, though carrying inconsistent data.
> Then we can even replay up to the end of WAL but always getting
> inconsistent data.
> BUT, you say, as the tuple is not commited in the WAL, only the old version
> of the tuple will be visible? Right?

Yes -- all the updated tuples are invisible because the commit record
for transaction 123 does not appear in wal. A future VACUUM will remove
all those tuples. Note that precisely for this reason, the original
version of the tuple had not been removed yet.

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