Re: Effective Oracle by Design - p259 - 260 - confused, is there a mistake?
- From: <t_adolph@xxxxxxxxxxx>
- To: "Mark W. Farnham" <mwf@xxxxxxxx>, "ORACLE-L" <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 2 Nov 2005 12:34:50 +0100
So far my understanding's matched all feedback, just can't match that with the
description/example in the book.
----- Original Message -----
From: Mark W. Farnham
To: t_adolph@xxxxxxxxxxx ; ORACLE-L
Sent: Wednesday, November 02, 2005 12:16 PM
Subject: RE: Effective Oracle by Design - p259 - 260 - confused, is there a
mistake?
It all depends on when Session B's query is started. If Session B's query
starts before at least one of the commits, then to preserve read consistency it
must either acquire and apply the relevant undo to create a block image
consistent with the time of the start of the Session B query or it must report
snapshot too old. A commit does not "throw away" undo, but rather releases it
as no longer required by an updating transaction. When and if "undo" is
actually discarded from all available sources grows increasingly complex with
flashback (not relevant to 9.2.x). It varies qualitatively from keeping it
until the space consumed limit is reached to a new mode that is akin logically
to redo being unable to wrap to the next log in that it will hang instead of
dying and will resume if you add space. Oracle has achieved a laudable balance
in making the default usage trivial to configure yet allowing you to tailor
behavior precisely to satisfy particular requirements.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of t_adolph@xxxxxxxxxxx
Sent: Wednesday, November 02, 2005 5:06 AM
To: ORACLE-L
Subject: Effective Oracle by Design - p259 - 260 - confused, is there a
mistake?
Hi All,
A question for those of you who have read Effective Oracle by Design by
Thomas Kyte:
Chpt 5, page 259 - 260: Tom is explaining that undo is read for read for
read consistency....
<snip>
Now this makes sense to me as in session A there was no commit. But in
Tom's pl/sql there's a commit every update. Shouldn't that throw away the undo
meaning that session B wouldn't be interested in it? I tried with a commit
every update and confirmed what I'd expected, only 4 gets. What have I missed
folks?
Tony
PS I thinks its irrelevant here, but Ora 9.2.0.7 on Win2k
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