Re: How are ITL entries banked?
- From: "vit\.spinka" <vit.spinka@xxxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Fri, 8 Jan 2010 18:01:15 +0100
Hi Martin,
the acutal algortihm is in fact a bit complicated; if I simplify it, then the
ITL is blanked by the DML session only if the block is still in the cache. This
is the "fast commit" cleanout.
If it's already on the disk, than it's blanked by any other session that reads
it from the disk next time. The session checks the undo if the referenced XID
is till active; if not, the commit SCN of that XID is used to blank the ITL. If
the XID is no longer in undo, we can safely assume the transaction already
commited and blank the ITL as well (using lowest SCN of any active
transaction). This is the "delayed block cleanout".
Note that blanking the ITL also involves setting the commit SCN of that
particular row, so we can properly answer any read-consistent queries.
Further details are described at http://www.jlcomp.demon.co.uk/cleanout.html .
Vit Spinka
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