Re: What's the dfifference between pg_start_backup+copy+pg_stop_backup+WAL vs. pg_start_backup+pg_stop_backup+copy+WAL?

Greetings,

* Thorsten Schöning (tschoening@am-soft.de) wrote:
> Guten Tag Scott Ribe,
> am Freitag, 23. Juli 2021 um 20:07 schrieben Sie:
>
> > Because it is in the file. WAL can re-do changes, it cannot undo them.
>
> But isn't visibility of rows associated to transaction numbers as
> well? I would have expected the transaction number for the row in B
> simply not being available, because it was never part of the WALs up
> to and including pg_stop_backup when not following the official
> ordering of commands. So I expected that row to simply not be visible
> at all and vacuumed at some point or whatever.

The information about what transactions have committed and what haven't
isn't just in the WAL- it's in the tuple header for frozen tuples, or
all visible tuples, and it's in the transaction log files otherwise, so
it would depend on when those are copied.

Thanks,

Stephen