Unindexed FK Cause Deadlock or Only Share Lock? 2005-06-30 - By Jonathan Lewis
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 (See ORA-00060.ora-code.com) 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
-- -- Original Message -- --
From: "Post, Ethan" <Ethan.Post@(protected)>
To: "Oracle-L@(protected) Org (E-mail)" <oracle-l@(protected)>
Sent: Thursday, June 30, 2005 7:32 PM
Subject: Unindexed FK Cause Deadlock or Only Share Lock?
I am trying to find out if a missing index on a foreign key would cause
a deadlock. I always thought is would only cause a share lock and hold
up other DML, not deadlock it.
This asktom link http://tinyurl.com/djgco (search for deadlock and read
comments/responses) seems to suggest you only get a share lock.
However I was sent these links also...some don't seem to support the
idea of deadlocks but others do. Anyone know the answer here? Anyone
think of a quick test case which demonstrates this?
--
http://www.freelists.org/webpage/oracle-l
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