Performance Tuning :: Query Hanging With Sequential Read Or Latch Free Waits?

Following query is hanging either with 'Sequential access read' or 'Latch Free' wait event Important thing is the table which is self joined in subquery here does not have any index at all While it was hanged I tried to get trace of it and terminated twice. As such haven't got 'row source generataion' The table has only 120000 records and it shall update 34000 records

UPDATE invoice_header inv
SET inv.modified_due_date =
(SELECT inv1.btn_due_date
FROM invoice_header inv1
WHERE inv.dct_code = inv1.dct_code AND inv1.release = 'A5')

[code]...

During 'sequential read' using p1,p2 values tried to get what the session is reading and found that it is using the table itself.

During lath free I found following
SELECT name, 'Child '||child#, gets, misses, sleeps
FROM v$latch_children
WHERE addr= (select p1raw from v$session_wait where sid=18)
UNION

[code]...

However instead of self join when I creaed global temporary table as

create global temporary table t as select * from invoice_header where release='A5'

And used it in the update as

UPDATE invoice_header inv
SET inv.modified_due_date =
(SELECT t.btn_due_date
FROM t
WHERE inv.dct_code = t.dct_code AND t.release = 'A5')
WHERE inv.release = 'A5' AND inv.btn_due_date >= TRUNC (SYSDATE)

It updated the records in a second!!

Questions are
1) why it is producing 'sequential read' wait event when there is no index access or else why it is doing single block access when FTS is required?
2) Why is the 'latch free' wait event here and what it indicates here with 'cache buffer handles'?
Is it because we are reading and updating the same segment?

know in case DDL of table is required. It has all nullable columns and no index at all. Since it is 9i I am unable to use MERGE effectively in this case