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Archives of the TeradataForum
Message Posted: Tue, 21 Jun 2011 @ 13:54:03 GMT
| Subj: | | Re: Distinct vs. group by in TD 13 onwards |
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| From: | | Dieter Noeth |
On the TD13 demo both version show the same plan, single-AMP aggregation, but adding that additional retrieve step :-(
Explanation
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| | 1) | First, we do a single-AMP SUM step to aggregate from au.trans by way of the primary index "au.trans.ACCOUNT_NUMBER = 1" with no residual
conditions, grouping by field1 ( au.trans.ACCOUNT_NUMBER). Aggregate Intermediate Results are computed locally, then placed in Spool 3. The size
of Spool 3 is estimated with high confidence to be 1 row (21 bytes). The estimated time for this step is 0.02 seconds.
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| | 2) | Next, we do a single-AMP RETRIEVE step from Spool 3 (Last Use) by way of the hash value of "au.trans.ACCOUNT_NUMBER = 1" into Spool 1
(group_amps), which is built locally on that AMP. The size of Spool 1 is estimated with high confidence to be 1 row (25 bytes). The estimated
time for this step is 0.02 seconds.
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