text: CREATE LUA SCALAR SCRIPT my_average
      (a DOUBLE, b DOUBLE ORDER BY 1 desc)
  RETURNS DOUBLE AS
      function run(ctx)
          if ctx.a == nil or ctx.b==nil
              then return NULL
          end
          return (ctx.a+ctx.b)/2
      end
  /
  CREATE LUA SCALAR SCRIPT my_average
      (a DOUBLE, b DOUBLE ORDER BY 1 desc)
  RETURNS DOUBLE AS
      function run(ctx)
          if ctx.a == nil or ctx.b==nil
              then return NULL
          end
          return (ctx.a+ctx.b)/2
      end
  /
  