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The new image iterator and delta iterator uses an iterator-based API. https://github.com/neondatabase/neon/pull/8006 / part of https://github.com/neondatabase/neon/issues/8002 This requires the underlying thing (the btree) to have an iterator API, and the iterator should have a type name so that it can be stored somewhere. ```rust pub struct DeltaLayerIterator { index_iterator: BTreeIterator } ``` versus ```rust pub struct DeltaLayerIterator { index_iterator: impl Stream<....> } ``` (this requires nightly flag and still buggy in the Rust compiler) There are multiple ways to achieve this: 1. Either write a BTreeIterator from scratch that provides `async next`. This is the most efficient way to do that. 2. Or wrap the current `get_stream` API, which is the current approach in the pull request. In the future, we should do (1), and the `get_stream` API should be refactored to use the iterator API. With (2), we have to wrap the `get_stream` API with `Pin<Box<dyn Stream>>`, where we have the overhead of dynamic dispatch. However, (2) needs a rewrite of the `visit` function, which would take some time to write and review. I'd like to define this iterator API first and work on a real iterator API later. ## Summary of changes Add `DiskBtreeIterator` and related tests. Signed-off-by: Alex Chi Z <chi@neon.tech>