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Archives of the TeradataForum
Message Posted: Tue, 23 Nov 2004 @ 18:43:00 GMT
| Subj: | | Explain - low vs high confidence |
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| From: | | akleema_talukder |
Hi all,
I have an SQL and the explain for this SQL is below. The column cust_id, and sbu_id has statistics and it is up-to-date. Then why it it is
showing low confidence? To get the high confidence what should I do?
Any hints will be greatly appreciated.
SQL:
explain
sel cust_id, sum(sbu_id) from gdyr_edw.cust group by 1;
Explain:
Explanation
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| | 1) | First, we lock a distinct gdyr_edw."pseudo table" for read on a RowHash to prevent global deadlock for gdyr_edw.cust.
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| | 2) | Next, we lock gdyr_edw.cust for read.
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| | 3) | We do a SUM step to aggregate from gdyr_edw.cust by way of an all-rows scan with no residual conditions, and the grouping identifier in
field 1025. Aggregate Intermediate Results are computed locally, then placed in Spool 3. The size of Spool 3 is estimated with low confidence to
be 380,547 rows.
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| | 4) | We do an all-AMPs RETRIEVE step from Spool 3 (Last Use) by way of an all-rows scan into Spool 1, which is built locally on the AMPs. The
size of Spool 1 is estimated with low confidence to be 380,547 rows. The estimated time for this step is 1.78 seconds.
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| | 5) | Finally, we send out an END TRANSACTION step to all AMPs involved in processing the request.
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| | -> | The contents of Spool 1 are sent back to the user as the result of statement 1.
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thanks,
Akleema
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