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Message Posted: Sat, 04 Jun 2011 @ 18:13:21 GMT

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Subj:   Re: Product Join: Duplicating large table
From:   DWellman


As well as the 'missing statistics' answer given previously, there is also 'stale statistics' to consider - although this should be less of a problem on TD 12+.

When you're referring to a "table being duplicated" (or "re-distributed" for that matter), remember that it is only qualifying rows that get duplicated/redistributed. Qualifying rows are those rows that meet any selection criteria in a WHERE / ON clause. So, even if the explain plan says that it is duplicatiing 'tableX' which is the large table in the query this may not be a bad plan.

Check how many rows are estimated to be in the resulting spool file. If a table has X million rows on it, but only 1000 rows qualify and other table has 500000 rows in it, then building a spool from 1000 rows is probably the better plan. But these 1000 rows come from 'the big table'.

Hope that is useful.



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