special integrity constraints

hi,

i have two tables

create table a (x integer);
create table b (y real);

i have the special constraint that the sum of the number of rows in
table a plus the number of rows in table b must be even.

so there is a posibility of adding one element to a and one element to
b, and again the constraint is met.

but this of course does not work, since between adding the element to a
and adding the element to b, the constraint is not met.

so i cannot use a trigger.

what i need is the execution of a check procedure at commit time. is
that somehow possible?

my database has much more tables than just a and b, and most time the other
tables are modified and not a or b, so it would be nice to execute the
constraint checking procedure only if a or b was modified.

yes :) this question i asked in my former mail too, but I think it
looked like there was a workaround and my problem was wrong. so i
created this new kind of problem here to make things clearer.

cu
Erik

--
Erik Thiele