Tx lock problem with bitmap index

Hi,
I am facing a possible oracle limitation in Oracle 8.1.6 (Solaris 7) in bitmap index. Unfortunately upgrade is not an option. The problem is as follows:

create table atest
(
a varchar2(10),
b number(10)
);
 I created a bitmap index on the column a. No Primary key or any constraint is present on the table. Now from two separate sessions, I issue the same insert statement:

insert into atest values('A',10);

If the first session does not commits, the second insert gets locked. Tracing shows only the insert is locked. Looks like the Bitmap index can not work with concurrent two inserts. But does not inserts should not be locked?
Recreating the indexes as B-tree index makes the problem go away. Any insights will be helpful.

Thanks and Regards,

Saikat Chakraborty

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