The WAL record of the heap insert/update/delete contains a flag
indicating that the visibility map needs to be updated too. Thus no need
for a separate WAL record.
Heikki,
Have you considered these cases?
a) The current WAL architecture makes sure that the WAL Log is written before the actual page flush( i believe ). But you are changing that architecture for Visibility maps. Visibility map might get flushed out before the corresponding WAL gets written. I think you would then suggest full page writes here
b) Say for a large table, you have multiple buffers of visibility map, then there is a chance that one buffer gets flushed to the disk and the other doesn't. If the WAL records are not in place, then this leads to a time inconsistent visibility map.
c) If you are going to track all the WAL linked with a buffer of visibility map, then you need to introduce another synchronization in the critical path.
May be i am missing something? I am asking these questions only out of curiosity.
Thanks,
Gokul.
