My apologies for missing that. One of these days I will summarize your
thoughts correctly. In the meantime, I appreciate your gracious attitude and
patience with the likes of me.
On 3/14/07, Wolfgang Breitling <breitliw_at_centrexcc.com> wrote:
>
> At 08:00 AM 3/14/2007, Charles Schultz wrote:
>
> >For instance, I was recently trading emails with Wolfgang Breitling,
> >and he was patiently clarifying his paper "Joins, Skews and
> >Histograms" (with kudos to Alberto Dell'Era). For an extremely
> >simple case (join a 20 row table to a 40 row table where half of the
> >rows from the first are repeated x4 in the latter), Wolfgang (and
> >Alberto) demonstrate that Oracle uses a highly complex algorithm to
> >calculate cardinality only when frequency histograms are used (as
> >opposed to SIZE AUTO or SIZE SKEW, or no stats). The kicker is that
> >the estimated cardinality is really bad (45% error).
>
> A small correction. The optimizer uses that complex algorithm every
> time there are histograms on the join columns, not just when there
> are frequency histograms. That is one of the main points. The
> algorithm doesn't distinguish between frequency and height-balanced
> histograms which for the referenced particular case results in the
> gross miscalculation.
>
>
> Regards
>
> Wolfgang Breitling
> Centrex Consulting Corporation
> www.centrexcc.com
>
>
>
--
Charles Schultz
--
http://www.freelists.org/webpage/oracle-l
Received on Wed Mar 14 2007 - 10:30:17 CDT
