Re: re "CBO - A Configuration Roadmap" -- Histograms on Non-Indexed Columns
- From: Hemant K Chitale <hkchital@xxxxxxxxxxxxxx>
- To: christian.antognini@xxxxxxxxxxxx, breitliw@xxxxxxxxxxxxx, mgogala@xxxxxxxxxxx
- Date: Fri, 29 Dec 2006 22:45:17 +0800
Christian, Wolfgang, Mladen,
I knew about cardinality driving execution plan if it is used to
determine the access method against the table. However, if there was
no index on the column, the only access on that column _standalone_
would be a full table scan. {exactly the first paragraph's in
the section on Histograms on Non-Indexed columns in
Wolfgang's paper , eh ?|
Now, from your emails and Christian's short example and
Wolfgang's paper, I take the explanation that this cardinality would
be used to determine the JOIN method (sort-merge , nested-loop or hash).
Thank you.
[[BTW : I read Christian's paper after encountering an issue with
a query on a single table [no join] where a column in the query
IS quite skewed but I thought that Christian's statement was a generalization
and began wondering how the Histogram would help in my query
{single table , no join}. I guess I was too quick to make that assumption. ]]
So, now, I should also consider skewness in a column if it is in a Join.
Hemant K Chitale
http://web.singnet.com.sg/~hkchital
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