| From: | Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, decibel <decibel(at)decibel(dot)org>, Greg Smith <gsmith(at)gregsmith(dot)com>, jd(at)commandprompt(dot)com, Grzegorz Jaskiewicz <gj(at)pointblue(dot)com(dot)pl>, Bernd Helmle <mailings(at)oopsware(dot)de>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: The science of optimization in practical terms? |
| Date: | 2009-02-18 19:46:32 |
| Message-ID: | 499C6598.9020604@cheapcomplexdevices.com |
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| Lists: | pgsql-hackers |
Robert Haas wrote:
> experience, most bad plans are caused by bad selectivity estimates,
> and the #1 source of bad selectivity estimates is selectivity
> estimates for unknown expressions.
ISTM unknown expressions should be modeled as a range of
values rather than one single arbitrary value.
For example, rather than just guessing 1000 rows, if an
unknown expression picked a wide range (say, 100 - 10000
rows; or maybe even 1 - table_size), the planner could
choose a plan which wouldn't be pathologically slow
regardless of if the guess was too low or too high.
For that matter, it seems if all estimates used a range
rather than a single value, ISTM less in general we would
product less fragile plans.
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