I am trying select contracts together with the contract lines. around 30.000 contracts and around 800.000 contract lines.  And I have two queries that to essentially the same (each contract has at least one contract_line):

select c.id
       c.contract_date,
       (select sum(cl.amount)
         from contract_lines cl
         where cl.contract_id=c.id)
  from contracts c
  where c.contract_date>='01.01.2015' and c.contract_date<='31.12.2015'
This query has few reads (around 10.000) but the execution time is very large. In fact, the execution time itself is negligible, but fetch time is very large. I guess - maybe Firebird is doing left join during execution time.

select distinct cl.contract_id
       sum(sl.amount)
  from contract_lines cl
    join contracts c on (cl.contract_id=c.id)
  group by cl.contract_id
This query has lot of reads (around 500.000) but the execution and fetch time is very small.

All the reads of both queries are indexed reads.

The question is - which query to choose? I am afraid of many reads, because they can create load on hard disk and that can slow down things for other users. But from the other side - practice and low execution time shows that I should use the second query.

Is large number of indexed reads harmful?

Jonatan