SQL Challenge/Puzzle: Given a stack trace - How to find the top element at each point in time?

Question

• My real life use-case was to merge nested ranges. I\'ve drew some sketches and then I saw the resemblance between ranges starting and ending to stack PUSH and POP oper

Answer 1

This is actually an interesting problem. What I would do is keep track of each elements position in the stack. You can do this using a cumulative sum:

select st.*,
       sum(case when op = 'I' then 1 else -1 end) over (order by i) as pos
from stack_trace st;

Alas, at this point, I think you need a rather complicated join or subquery to figure out the most recent value that pos refers to. Here is one method:

with st as (
      select st.*,
             sum(case when op = 'I' then 1 else -1 end) over (order by i) as pos
      from stack_trace st
     )
select st.*,
       (select val
        from st st2
        where st2.i <= st.id and st2.pos = st.pos and
              st2.val is not null
        order by i desc
        fetch first 1 row only
       ) as top_of_stack_val
from st;