RE: Unindexed FK Cause Deadlock or Only Share Lock?

I was hoping one of the more brilliant scienticians from the other side
of the pond would chime in. If I may beg you to waste a little more time
with my humble request. What happens when there is an index, and let's
assume the index only contains 2 distinct values over zillions of
records. Have we eliminated the deadlock scenario below entirely? I am
all for indexing my FK's but was not aware such a otherwise useless
index (as described in previous sentence) could be so useful?

-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Jonathan Lewis
Sent: Thursday, June 30, 2005 3:21 PM
To: Oracle-L@Freelists. Org (E-mail)
Subject: Re: Unindexed FK Cause Deadlock or Only Share Lock?


If you attempt to delete a parent row,
or update the key value of a parent
row in a parent/child relationship where
the child DOES NOT HAVE an index 
on the foreign key, then Oracle will
attempt to acquire a mode 4 lock
on the child table (or mode 5 if it
has previously modified the child
table).

If any other sessions are currently
modifying the child table, your session
will have to wait before it can acquire
its mode 4/5 as the other sessions will
be holding mode 3 and therefore will
be blocking you.

Anyone who tries to start a new transaction
on the child table (and therefore need to
acquire a mode 3) will be blocked behind
your request for a mode 4.


To engineer a deadlock:
    session A
        delete child row C1
        acquires mode 3 on child table

    session B
        delete child row C2
        acquires mode 3 on child table

    session A
        attempts to delete parent of C1
        attempts to convert mode 3 to mode 5
        blocked by session B holding mode 3
            therefore starts to wait

    session B
        attempts to delete parent of C2
        attempts to convert mode 3 to mode 5
        blocked by session A in the converters queue
            (viz: holding 3, and waiting to convert to 5)
            therefore start to wait

    Three seconds or less later, session a gets
    an ORA-00060 Deadlock detected.



Regards

Jonathan Lewis

http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ

http://www.jlcomp.demon.co.uk/appearances.html
Public Appearances - schedule updated June 22nd 2005


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