Re: what's the internal transaction table for....
Date: Mon, 8 Feb 2010 19:29:00 -0800 (PST)
Message-ID: <585d7913-4a10-4de5-a331-c7c8348bd39b_at_m4g2000vbn.googlegroups.com>
On Feb 8, 4:03 pm, Mark D Powell <Mark.Powe..._at_hp.com> wrote:
> On Feb 8, 2:53 pm, Ken Quirici <kquir..._at_yahoo.com> wrote:
>
>
>
>
>
> > The 10g discussion of transaction commitment has this:
>
> > The internal transaction table for the associated undo tablespace that
> > the transaction has committed, and the corresponding unique system
> > change number (SCN) of the transaction is assigned and recorded in the
> > table.
>
> > which the 11g manual corrects to:
>
> > The internal transaction table for the associated undo tablespace
> > RECORDS [my uppercase] that the transaction has committed, etc.
>
> > What is this internal transaction table? I can't find reference to it
> > anywhere else. It seems as though it
> > would be important in deciding how long to keep rollback segments
> > around, but I could be wrong there,
> > since they seem to be kept around forever, or until they get
> > overwritten, whichever comes first.
>
> > Thanks for any illumination!
>
> I believe that the material is referring to the Interested Transaction
> List, ITL, stored in each table block to keep track of changes by
> pointing to the undo segment that contains the undo for the
> transaction. The table parameter initrans controls how many of these
> areas are pre-allocated to the blocks. Each ITL is 23 bytes in length
> plus I believe the ITL is preceeded by a length or usage byte so the
> cost is 24 bytes each space wise.
>
> Jonathan Lewis has written in detail on Oracle usage of the ITL. You
> can find his web site via a search.
> You can see the ITL in a block dump.
>
> HTH -- Mark D Powell --
I think what the documentation I was quoting was referring to was
something
stored in the rollback segments. There was an illustration which
showed
Oracle ensuring query data consistency by getting blocks from rollback
segments with SCN's less than (or equal to except it only illustrated
SCN's less than) what was the current SCN when the query started. It
can only be these internal transaction tables, it seems, that have
this
SCN *in* the rollback segments.
And I also think Oracle 'blindly' overwrites rollback segments when it
gets to the end of allocating new rollback segments up to the amount
specified in the init parameters as max. There appears to be no
intelligence involved in the overwrite or not-overwrite process
except,
if you run out of space, start from the beginning again.
This is much less sophisticated than I had thought it might be. ON the other hand, it works nearly all the time! Received on Mon Feb 08 2010 - 21:29:00 CST