Re: how to determine number of chained rows
- From: Tanel Poder <tanel@xxxxxxxxxxxxxx>
- To: JBECKSTROM@xxxxxxxxx
- Date: Tue, 16 Apr 2013 00:06:16 +0300
One way to estimate the impact of chained rows is to just look into the
"table fetch continued row" statistic - you can run your query/workload and
measure this metric from v$sesstat (with snapper for example).
And one more way to estimate the total number of chained pieces would be to
run something like SELECT /*+ FULL(t) */ MIN(last_col) FROM t and see how
much the "table fetch continued row" metric increases throughout the full
table scan. The last_col would be the (physical) last column of the table.
Note that if your wide row is chained into let's say 4 pieces, then you'd
see the metric increase by 3 for a row where 4th row piece had to be
fetched.
I've written about this here:
http://blog.tanelpoder.com/2009/11/04/detect-chained-and-migrated-rows-in-oracle/
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*Tanel Poder*
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On Mon, Apr 15, 2013 at 5:57 PM, Jeffrey Beckstrom <JBECKSTROM@xxxxxxxxx>wrote:
> DBMS_STATS does not compute number of chained rows.
> analyze table x list chained rows - this does but if the table is large
> and has lots of chained rows, then need a lot of rollback space for rows
> inserted into the chained_rows table.
>
> Is there another way to just get a count of chained rows?
>
>
> Jeffrey Beckstrom
> Database Administrator
> Greater Cleveland Regional Transit Authority
> Information Systems
> 1240 W. 6th Street
> Cleveland, Ohio 44113
>
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