create table T_mqt as (select count(field) cnt from T)
data initially deferred refresh deferred maintained by user 
in userspace1;

create trigger T_ais 
after insert on T
referencing new table as n
for each statement
update T_mqt set cnt = cnt + (select count(field) from n);

create trigger T_ads 
after delete on T
referencing old table as n
for each statement
update T_mqt set cnt = cnt - (select count(field) from n);

set integrity for T_mqt materialized query immediate unchecked;
insert into T_mqt select count(field) from T;
-- 20 минут покурите или чаю попейте

runstats on table your_schema.T_mqt;

set current refresh age ANY;
set current maintained types SYSTEM, USER;