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A key to delivering high-performance scientific data stores at multi-petabyte scales and beyond is to provide an infrastructure that allows scientists to select, read, and process only the data that they need. The ATLAS experiment at the Large Hadron Collider (LHC) has deployed an event data store and an I/O framework designed from their inception to support efficient selective reading at many scales, from event-level selections to partial retrieval of the complexly-structured event data objects themselves. In-file metadata is used to accomplish the bookkeeping needed to accompany these data reduction strategies. This paper describes features of the ATLAS I/O and event data store infrastructure that make efficient selective reading possible.