Re: A thought on Index Organized Tables

> Yes. The visibility map doesn't need any new WAL records to be written.
>
> We probably will need to add some WAL logging to close the holes with
> crash recovery, required for relying on it for index-only-scans, but
> AFAICS only for VACUUM and probably only one WAL record for a whole
> bunch of heap pages, so it should be pretty insignificant.
>

Hmmm.... So whenever the update transaction changes a page, it will update
the visibility map, but won't enter the WAL record.
So after the crash we have a visibility map, which has false positives.
Isn't that wrong?

>
> Let me repeat myself: if you think the overhead of a visibility map is
> noticeable or meaningful in any scenario, the onus is on you to show
> what that scenario is. I am not aware of such a scenario, which doesn't
> mean that it doesn't exist, of course, but hand-waving is not helpful.
>

Well as a DB Tuner, i am requesting to make it a optional feature. If you
and everyone else feel convinced, consider my request.

>
>
> I'm not sure what you mean with "without any page level locking".
> Whenever a visibility map page is read or modified, a lock is taken on
> the buffer.
>
>
OK. I thought you are following some kind of lock-less algorithm there.
Then updaters/deleters of multiple pages might be waiting on the same lock
and hence there is a chance of a contention there right? Again correct me,
if i am wrong ( i might have understood things incorrectly )

Thanks,
Gokul.