RE: Oracle Read Consistent Overhead
- From: Matt McClernon <mccmx@xxxxxxxxxxx>
- To: <jonathan@xxxxxxxxxxxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 18 Apr 2011 00:59:47 +0000
>
> How large was the "medium-sized" transaction.
I just repeated the test again the and medium sized transaction was 100,000
rows (update of one column)
> Read-consistency costs are largely about the number of undo records applied,
> not
> about the number of blocks in the underlying object, and the number of undo
> records is (generally) related to the number of changes, which often means
> number of rows.
Interesting. I did suspect that the multi-versioning might be row based, not
block based, but I ruled it out because it seemed too inefficient.
In my re-run of the simulation today I saw 200,000 consistent reads in the
second session which is 2 CR blocks per row update. That still seems a little
high.
More interesting than that though is that I repeated the test with an index on
the MV log and the CR count was 3.8 million..! which is 38 CR blocks per row
update. That is highly suspicious.
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