Michał Chojnowski 55c4b89b88 sstables: make sstable::estimated_keys_for_range asynchronous ...

Currently, `sstable::estimated_keys_for_range` works by
checking what fraction of Summary is covered by the given
range, and multiplying this fraction to the number of all keys.
Since computing things on Summary doesn't involve I/O (because Summary
is always kept in RAM), this is synchronous.

In a later patch, we will modify `sstable::estimated_keys_for_range`
so that it can deal with sstables that don't have a Summary
(because they use BTI indexes instead of BIG indexes).
In that case, the function is going to compute the relevant fraction
by using the index instead of Summary. This will require making
the function asynchronous. This is what we do in this patch.

(The actual change to the logic of `sstable::estimated_keys_for_range`
will come in the next patch. In this one, we only make it asynchronous).

2025-09-29 13:01:21 +02:00

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