So, is it safe to assume that there is no documentation for the exact content of the block index?
I think that reading the source code should be the last resort, since one cannot sure whether it is an implementation detail, or it is the specification that can be relied upon.
The information on the exact content of the block index is important, since it is related in the size of the index (let alone the query performance) and therefore the schema design.

Or, could you please simply provide the exact answer to my original question(with proper reference)?

Thank you.

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