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:
