fn pick_l1_candidate_files(
l1_files: Vec<FileHandle>,
max_output_file_size: Option<u64>,
phase: PickPhase,
mapper: &PrimaryKeyRangeMapper,
) -> (Vec<FileHandle>, usize)fn pick_l1_candidate_files(
l1_files: Vec<FileHandle>,
max_output_file_size: Option<u64>,
phase: PickPhase,
mapper: &PrimaryKeyRangeMapper,
) -> (Vec<FileHandle>, usize)