Oracle FAQ Your Portal to the Oracle Knowledge Grid
HOME | ASK QUESTION | ADD INFO | SEARCH | E-MAIL US
 

Home -> Community -> Mailing Lists -> Oracle-L -> Re: Read Consistency & ITL's

Re: Read Consistency & ITL's

From: <Jared.Still_at_radisys.com>
Date: Tue, 05 Mar 2002 10:13:28 -0800
Message-ID: <F001.0041FC82.20020305101328@fatcity.com>


I'll take the easy question.

There's nothing preventing you from granting privs on the x$ tables.

See:
  http://www.ixora.com.au/scripts/sql/create_xviews.sql

Jared

Rajesh.Rao_at_jpmchase.com
Sent by: root_at_fatcity.com
03/04/02 04:53 PM
Please respond to ORACLE-L  

        To:     Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
        cc: 
        Subject:        Read Consistency & ITL's


Hello Gurus,

As I understand it. In order to provide a read consistent view of the data while reading a data block, Oracle looks at the SCN in the block header, and compares it to the snapshot of the SCN taken when the read commenced. If the Snapshot SCN is less than the SCN in the block header, the query is directed to read from the rollback segments.

For any transaction that modifies a block, the ITL among other things, also
stores the commit SCN and the address to the transaction table in the rollback segment. Assume that the block has just one ITL. This ITL can be reused once the transaction is completed. Assume it is. That is, two transactions have performed updates on the block since our read commenced. If so is the case, how does Oracle know which rollback segment to look at? I am assuming it still looks at the ITL, rolls it back, sees that it needs to rollback further, looks at the ITL in the rolled back block, and rollsback further, and so on, until it can reconstruct the data block at an
SCN lower than the snapshot SCN. Is that right?

My second question is, what happens if the data block has two ITL's, both marked with SCN's greater than when the read commenced. Which ITL does Oracle look at to get the address of the rollback segment? Is it the one with the least SCN??

And also, my previous question, remains unanswered so far? Why does Oracle not allow one to grant select privileges on the fixed tables to any other user?

Thanks
Raj

--

Please see the official ORACLE-L FAQ: http://www.orafaq.com
--

Author:
  INET: Rajesh.Rao_at_jpmchase.com

Fat City Network Services    -- (858) 538-5051  FAX: (858) 538-5051
San Diego, California        -- Public Internet access / Mailing Lists
--------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).

--

Please see the official ORACLE-L FAQ: http://www.orafaq.com
--

Author:
  INET: Jared.Still_at_radisys.com

Fat City Network Services    -- (858) 538-5051  FAX: (858) 538-5051
San Diego, California        -- Public Internet access / Mailing Lists
--------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). Received on Tue Mar 05 2002 - 12:13:28 CST

Original text of this message

HOME | ASK QUESTION | ADD INFO | SEARCH | E-MAIL US