Desafio/Quebra-cabeça SQL:Dado um rastreamento de pilha - Como encontrar o elemento superior em cada ponto no tempo?

SELECT dt.*,
   -- find the last item in the stack with the same position
   Last_Value(val IGNORE NULLS)
   Over (PARTITION BY pos
         ORDER BY i) AS top_of_stack_val
FROM 
 ( 
   SELECT st.*,
      -- calculate the number of items in the stack
      Sum(CASE WHEN op = 'I' THEN 1 ELSE -1 end) 
      Over (ORDER BY i
            ROWS Unbounded Preceding) AS pos
   FROM stack_trace AS st
 ) AS dt;