Function open_projected_reader 

pub async fn open_projected_reader<OpenIndexed, IndexedFuture, OpenFull, FullFuture>(
    version: ConcreteFileVersion,
    projection: &ReaderProjection,
    prefer_indexed: bool,
    open_indexed: OpenIndexed,
    open_full: OpenFull,
) -> Result<ProjectedFileReader>
where
    OpenIndexed: FnOnce() -> IndexedFuture + Send,
    IndexedFuture: Future<Output = Result<Option<ProjectedFileReader>>> + Send,
    OpenFull: FnOnce() -> FullFuture + Send,
    FullFuture: Future<Output = Result<ProjectedFileReader>> + Send,

Open a projected reader while keeping exact metadata-form selection in the file layer.

open_indexed returns None when the loaded index is not selective enough to justify a projected reader. V2.0 never invokes it because indexed metadata is not part of that reader’s accepted grammar.