Carlos,
you're right in that it doesn't sound very sensible that setting
RecordCountAccurate or AutoFetchAll should solve this problem, but I
am surprised that your query is allowed!

I thought that with a left join only the table(s) to the left of the
join where allowed in the where clause and that a query like yours
would have to be rewritten

select p.Cod_MatBibl, e.Num_Ex, p.Per_Titulo, e.Per_Volume,
e.Per_Fasc, e.Per_MesPub, e.Per_AnoPub
from Periodic p
left outer join Exemplar e on (p.Cod_MatBibl=e.Cod_MatBibl) and
(e.Ex_IndEtiq='S')
where (P.Cod_Matbibl >= :inicio) and P.Cod_Matbibl <= :fim)
union
select cast (9999999 as integer) as Cod_MatBibl,
cast(0 as smallint) as Num_ex,
f_strblob(' ') as Per_Titulo,
cast(0 as smallint) as Per_Volume,
cast('' as varchar(10)) as Per_Fasc,
cast('' as char(3)) as Per_MesPub,
cast(0 as smallint) as Per_AnoPub
from rdb$database

Does it still produce the error if you do this?

Set