>> Well, that would certainly be alarming if true, but I don't think it
>> is.  As far as I can see, the overhead of making the visibility map
>> crash-safe is just (1) a very small percentage increase in the work
>> being done by VACUUM and (2) a slight possibility of extra work done
>> by a foreground process if the visibility map bit changes at almost
>> exactly the same time the process was about to insert, update, or
>> delete a tuple.
>> Let's forget the overhead posed by vacuum. Can you please point me to the
> design which talks in detail of the second overhead?
> Thanks.

If you are following the same design that Heikki put forward, then there is
a problem with it in maintaining the bits in page and the bits in visibility
map in sync, which we have already discussed.

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