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Archives of the TeradataForum
Message Posted: Thu, 28 Sep 2006 @ 21:43:58 GMT
| Subj: | | Re: Explain Plan Question |
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| From: | | Anomy Anom |
<-- Anonymously Posted: Thursday, September 28, 2006 17:33 -->
| | I am running a query where I have identical NUPI indices on two tables. Each index is 3 columns. I have collect statistics on each
column in the index as well as the index itself. When I run an explain plan, it tells me that it is estimating the output with low confidence
(see below). I would have expected this to be high confidence since I have collect statistics. | |
| | Can anyone explain this to me? | |
I think the Optimizer knows exactly how many NULLs you have in each one of the two NULLable columns in the NUPI for both tables, but it is not
able to estimate accurately how many rows will satisfy the residual conditions applied to both columns in each row processed by the Merge Join,
both on the left (alias a) and on the right (alias b). I guess these conditions have been explicitly generated by the Parser, they are not present
in your SQL code.
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