Transactions and unique constraint

Hi everyone,
I have a problem with 2 inherited tables and a primery key constraint.

table a ( id serial primary key );
table b () inherits (a);
alter table b add constraint

If I delete one record of a, and then insert the same values into b, there is
an error telling me there is a duplicate key violating the unquie key
constraint.

This 2 queries are following directly each other in my PHP code. If I run the
2nd query (which inserts the values) afterwards with psql, it works. It seems
like Postgres needs some time recognizing that the primary key isn't used any
more. These queries are part of a transaction.

Any hints how I get this working?

Thanks,
Verena