B-tree on-disk anatomy: the metapage, the levels, and how a range scan moves
Simple terms
Picture the index as a stack of fixed-size pages. One page at the front is just a signpost: here is the top of the tree, and here is how tall it is. Everything in the middle is directions, pointers down to other pages. Only the bottom row holds real entries. Three kinds of page, and that is the whole structure. Those bottom pages are also chained left to right, which is the entire reason a range scan can walk sideways instead of climbing back up the tree for every row it wants.
You might be asked
Walk me through what a PostgreSQL B-tree index actually looks like on disk. What is in the metapage, how are the levels arranged, how does a range scan move across leaves, and what is the 'high key' for?
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