Function flat_projected_columns 

pub(crate) fn flat_projected_columns(
    metadata: &RegionMetadata,
    format_projection: &FormatProjection,
    json_target_types: &Arc<BTreeMap<ColumnId, JsonNativeType>>,
) -> Vec<(ColumnId, ConcreteDataType)>

Returns ids and datatypes of columns after applying the projection and JSON2 target types.

It adds the time index column if it doesn’t present in the projection.