Re: checkpoint clarifications needed

On 01/09/2017 01:10 PM, Tom DalPozzo wrote:
> Reread your original post and realized you where also asking
> about transaction consistency and WALs. The thumbnail version is
> that Postgres writes transactions to the WALs before they are
> written to the data files on disk. A checkpoint represents a
> point in the sequence when is is known that the changes recorded
> in the WAL have been also recorded in the disk data files. So
> Postgres then knows that in a recovery scenario it needs to only
> redo/replay the WAL changes that are past the last checkpoint.
> So the transactions are there it is just a matter of if they
> need to be replayed or not. This is subject to caveats:
>
>
> https://www.postgresql.org/docs/9.5/static/wal-reliability.html
> <https://www.postgresql.org/docs/9.5/static/wal-reliability.html>
>
>
> Hi, I had already read that doc but I can't answer clearly to my
> questions 2,4 and 5.

The answer would seem to depend on what you consider 'a consistency
state position'. Is it possible to be more explicit about what you mean?

> Regards
> Pupillo
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com