RE: delayed block cleanout (?) for active tx

From: Yasin Baskan <yasin.baskan_at_yapikredi.com.tr>
Date: Thu, 21 Jun 2007 16:39:05 +0300
Message-ID: <083667B535F3464CA0DD0D1DAFA4E3760DB59E18@camexc1.kfs.local>

I think what causes the redo is this:

"Suppose the ITL is still open and the transaction is not yet committed:

A reader on finding the ITL will create a consistent read copy of the block because a view of the block before the change was made is required. To make a CR copy, the undo from the rollback segment is applied to he block."

http://www.geocities.com/kgkrish/transactions.html

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Alberto Dell'Era Sent: Thursday, June 21, 2007 3:29 PM
To: Oracle-L
Subject: delayed block cleanout (?) for active tx

Dear all,

create table t as select rpad(rownum, 100) x from dual connect by level <= 100000;
update t set x=x;

in another session:
set autotrace on
select count(x) from t;
Statistics

...
157612 redo size

If I select again - same redo size:

select count(x) from t;
Statistics

...
157612 redo size

Notice I haven't committed the update, so this is not the classic redo generated due to classic delayed block cleanout (updates to the ITL to mark the tx entries as "committed").

It is anyway related to cleanout, in fact v$mystat shows entries such as:

active txn count during cleanout                       3031
cleanout - number of ktugct calls                     3031
immediate (CR) block cleanout applications      3031
db block changes                                           3031

cleanouts and rollbacks - consistent read gets 3031

Clearly the select has to build the consistent image of the blocks, and clearly it has to visit the ITL, and then the rollback segment, to see whether the tx has committed etc etc; then, it seems that it updates "something" in the block (db block changes). Does anybody know what this "something" is, assuming of course my reasoning so far is correct ?

9.2.0.8 on Windows XP.

Thanks in advance
Alberto

-- 
Alberto Dell'Era


"the more you know, the faster you go"

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Received on Thu Jun 21 2007 - 08:39:05 CDT