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  DECLARE
    selected_row jsonb;
    second_set_of_selected_rows jsonb;
    third_set_of_selected_rows jsonb;
    the_result jsonb;
BEGIN

    SELECT jsonb_agg(to_jsonb(x))
    FROM (
        SELECT
        jsonb_build_object('id', table1.id,
                   ...)
        FROM some_table table1
        WHERE id = id_sent_to_function
        GROUP BY table1.id
    ) x INTO selected_row;

    SELECT jsonb_agg(to_jsonb(y))
    FROM (
        SELECT
        jsonb_build_object('id', table_2.id,
                   ...)
        FROM another_table table_2
        JOIN a_map_table amt ON table_2.id = amt.some_id
          AND amt.second_id = id_sent_to_function
        GROUP BY table_2.id, amt.p::DOUBLE PRECISION / amt.q::DOUBLE PRECISION
        ORDER BY some_field
    ) y INTO second_set_of_selected_rows;

    SELECT jsonb_agg(to_jsonb(z))
    FROM (
        SELECT jsonb_build_object('id', table_3.id,
                          ...)
        FROM process table_3
        WHERE table_3.id NOT IN (
            SELECT xyz.id
            FROM second_map_table smt
            WHERE smt.methodology_id = id_sent_to_function)
        GROUP BY table_3.id
        ORDER BY another_field
    ) z INTO third_set_of_selected_rows;

    the_result =
        jsonb_build_object(
                           'selected_row', selected_row,
                           'second_set_of_selected_rows', second_set_of_selected_rows,
                           'third_set_of_selected_rows', third_set_of_selected_rows
                          );

RETURN RESULT;
END;