with a_ver(cur, val, dt_from, dt_till) 
  as ( select cur, val, dt dt_from
            , lead(dt,1,date'3000-01-01') over(partition by cur order by dt)-1/86400 dt_till
         from a )
select b.cur, b.dt, a.val
from b left join a_ver a on a.cur = b.cur and b.dt between a.dt_from and a.dt_till
;

select b.cur, b.dt, max(a.val) keep (dense_rank last order by a.dt) val
from b left join a on a.cur = b.cur and b.dt >= a.dt
group by b.cur, b.dt
;

select b.cur, b.dt, a.val
from b, lateral(select val from a where a.cur=b.cur and a.dt <= b.dt order by a.dt desc fetch first 1 row only) a 
;

select b.cur, b.dt, (select val from a where a.cur=b.cur and a.dt <= b.dt order by a.dt desc fetch first 1 row only) val
from b  
;

with uni as (
select 'a' g, cur, dt, val from a 
union all
select 'b', b.cur, b.dt, null val from b  
)
select *
from uni 
match_recognize(
partition by cur
order by dt desc, g desc
measures first(b.dt) as dt, a.val as val
after match skip to next row
pattern (b+ a)
define b as b.g='b', a as a.g='a'
)
;


with uni as (
select 'a' g, cur, dt, val from a 
union all
select 'b', b.cur, b.dt, null val from b  
)
select cur, dt, val 
  from ( select g, cur, dt, lag(val ignore nulls) over(partition by cur order by dt, g) val
           from uni)
where g='b'
order by cur, dt
;