Re: seen a Select (just a select no select for update) take out a Table Exclusive lock (TX)
- From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 26 Jan 2006 22:04:02 -0000
We need more details about the TX enqueue -
it's a transaction enqueue, not a table enqueue
by the way, and although they most commonly
appear because you have changed data, assuming
that this is the case may take you looking in the
wrong direction.
One reason why a session executing a select state
might attempt to acquire a TX enqueue is if the
table in question is subject to distributed transactions,
hence two-phase commit.
Between the prepare and commit of a two phase commit,
the state of a data block should be treated as indeterminate,
so any query that starts in that time-window should not be
allowed to see the old version of the block in case the
distributed transaction has successfully committed, nor should
it be allowed to see the new version of the block in case the
distributed transaction has rolled back - so it has to acquire a
TX mode 4 (share) lock on the TX slot of the distributed transaction
and wait for the two-phase commit to complete.
NB Prior to about 9.2.0.6, this was the behaviour exhibited by
Oracle in all cases of distributed transactions. In 9.2.0.6, Oracle
rewrote the code, and a query against a block modified by an
incoming XA transaction gets to see the earlier version of the block,
although an Oracle-only incoming transaction still results in the
enqueue appearing.
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/cbo_book/ind_book.html
Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/appearances.html
Public Appearances - schedule updated 10th Jan 2006
----- Original Message -----
From: "Wolfson Larry - lwolfs" <lawrence.wolfson@xxxxxxxxxx>
Cc: <oracle-l@xxxxxxxxxxxxx>
Sent: Thursday, January 26, 2006 9:22 PM
Subject: seen a Select (just a select no select for update) take out a Table
Exclusive lock (TX)
From one of the other DBAs here:
"Have any of you ever seen a Select (just a select no select for update)
take out a Table Exclusive lock (TX) ? we had one in SAP this morning on
ZPICK01 and the developers assure me that the transaction doesn't try to
update the table. The only thing I can think of, is that ZPICK01 is huge
and that Oracle tried to ensure some form of Read Consistency by making
the table TX. That said I'm still not certain that some switch in the
transaction isn't set to lock the table. Thoughts ?"
Gee, I haven't worked much on SAP lately, but I believe tables prefixed
Y and Z are custom tables, although I don't see why that would make a
difference.
Thanks
Larry
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