Oracle Explicit authorization

Same Database
two different users user1 User2

user1 there is a table table1

create synonym inside the user2
create synonym sy1 for user1.table1;

once created, direct access is OK.

select * from Sy1;

data can be taken.

but put it in a stored procedure.

CREATE PROCEDURE "Proc_test"
is
cursor Cur_test is
SELECT * from sy1 where rownum<10;
begin
open cur_test;
close cur_test;
end;

will error, said the table sy1 does not exist.

But if it is a different database, the establishment of such a synonym, but it is possible.

Really do not understand why this is so, please master enlighten.

As below, where CustomerInfo is the table name of the current database, the to is followed by the user to assign permissions to.

GRANT all on CustomerInfo to Esb_order

Oracle Explicit authorization