Default on update

Hi.
Is there a way to create "default" constraint on UPDATE query.
It's becouse I have a bool field that may NOT support NULL value, but the
Front-End calls null for FALSE values.
I was thinking something like:create table table1 ( id serial primary key, bv bool default false not null);
I would want to replace "bv" values with FALSE when insert/update NULL value for
this field.
Or need I create a TRIGGER that check it and replace the itens???
CREATE or REPLACE function TG_table1_check RETURNS trigger as 'BEGIN IF nullvalue(NEW.bv) THEN  NEW.bv=FALSE; END
IF;END;
' language 'plpgsql'; CREATE TRIGGER TG_table1_check BEFORE UPDATE on table1 for
each row execute procedure tg_table1_check();

Other question: I have a lot of triggers in my db system, I have table that has
5/6 triggers, many triggers are simple (like the tg_table1_check), any are
complex... Is it a problem??? My tests are with few regs and run fine. Where
can I read more about triggers and performance?

Thanks for all.
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Lucas Vendramin