Re: index prefetching

h, right. Well, you're right in this case we perhaps could set just one

of those flags, but the "purpose" of the two places is quite different.

The "prefetch" flag is fully controlled by the prefetcher, and it's up
to it to change it (e.g. I can easily imagine some new logic touching
setting it to "false" for some reason).

The "data" flag is fully controlled by the custom callbacks, so whatever
the callback stores, will be there.

I don't think it's worth simplifying this. In particular, I don't think
the callback can assume it can rely on the "prefetch" flag.

Why not to add "all_visible" flag to IndexPrefetchEntry ? If will not cause any extra space overhead (because of alignment), but allows to avoid dynamic memory allocation (not sure if it is critical, but nice to avoid if possible).