RE: Understanding Dynamic Sampling

Hi Ray

> I understand how dynamic sampling would assist in the
> scenario described in Level 4, but can someone help me
> understand "all tables for which standard selectivity
> estimation used a guess for some predicate that is a
> potential dynamic sampling predicate" ?

The following two queries provide an example.

SELECT *
FROM t
WHERE id = 19

SELECT *
FROM t
WHERE round(id) = 19

For the first one the query optimizer is able to estimate the selectivity of the predicate id=19 based on the column statistics. Thus no dynamic sampling is necessary. Instead for the second one without dynamic sampling the query optimizer should guess the selectivity of the predicate round(id)=19. In fact the column statistics only provide information about the column id itself, and not about its rounded values.

HTH
Chris

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