Different Lock Behavior With Create and Drop Foreign Key

Hi Team,
Here is test case.----create table testx
(
    v_code character varying(32),
    client_id bigint
);
alter table testx add constraint testx_pkey primary key (v_code);

create table testy
(
    dxid bigint,
    v_code character varying(32)
);
alter table testy add constraint testy_pkey primary key (dxid);create index on testy (v_code);

Let's begin two session, in session1 I am going begin a transaction and run select on parent table:
Session1----begin;select * from testx;

On another session let's call session2, I am running create FK on second table
Session2--- alter table testy add constraint testy_fkey foreign key (v_code) references testx(v_code); <--This works.alter table testy drop constraint testy_fkey; <--Hangs


Regards,Virendra Kumar