{"_yql_row_spec"={
    "Type"=["StructType";[
        ["event_dt";["OptionalType";["DataType";"String"]]];
        ["key";["OptionalType";["DataType";"String"]]];
        ["subkey";["OptionalType";["DataType";"String"]]];
        ["value";["OptionalType";["DataType";"String"]]]
    ]];
}}