>
> 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
>
>
>> Hi, I had already read that doc but I can't answer clearly to my
questions 2,4 and 5.
Regards
Pupillo