Re: Oracle Data Guard

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Thu, 11 Dec 2003 00:49:26 -0800
Message-ID: <F001.005D978A.20031211004926@fatcity.com>

But the last time I looked at it, you had to enable supplemental logging at the
database level if you wanted to use logical standby. Two side effects -

As you said, you need a unique/primary key, and database supplemental logging copies such a key into the redo for every change to a row: but if there is not uk/pk, then the whole row is copied.
The copy into redo is engineered by copying into the UNDO first - and since changes to GTTs are recorded into the UNDO, this means you get an extra volume of UNDO, hence REDO on all changes to 'supposed to be low-cost' GTTs.

The other feature of logical standby is that Oracle scrapes the redo log to generate LCRs (logical change records) which are then checked against your 'Streams rule-sets' - and then written into the local database for propagation to the remote via AQ mechanisms.

The overheads could be quite significant.

Regards

Jonathan Lewis
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Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Tuesday, December 09, 2003 9:59 PM

Hi Vi,

Rows NEED unique identification. So, if there are bunches of raw data with no unique identifier whatsoever (remember, rowid is not allowed) LSB can't generate a where-clause what row to update or delete on the SB database. It's generating SQL based on redolog info, and has to come up with an UPDATE <table> SET .... WHERE <unique id> = <unique id>. The unique id may be a multi-column key. There is an escape. Enabling supplemental logging can add extra info to do the unique identification, when no usable keys are available. This will cause some extra logging to be generated, there ain't no such thing as a free lunch. For detailed information read chapter 4.1.5 & 4.1.6 in the Oracle Data Guard Concepts and Administration manual, part no. A96653-02.

regards, Carel-Jan
At 15:54 8-12-03 -0800, you wrote:

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Author: Jonathan Lewis
INET: jonathan_at_jlcomp.demon.co.uk

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