| From: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
|---|---|
| To: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
| Cc: | Greg Stark <stark(at)mit(dot)edu>, decibel <decibel(at)decibel(dot)org>, Josh Berkus <josh(at)agliodbs(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Multi-pass planner |
| Date: | 2013-04-19 22:52:07 |
| Message-ID: | CAGTBQpaFQaO9dTNWoTZcn_yyr+HPO-iP7NX_d5LpaVwP+aH4Ug@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Fri, Apr 19, 2013 at 7:43 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> On Fri, Apr 19, 2013 at 2:24 PM, Claudio Freire <klaussfreire(at)gmail(dot)com>
> wrote:
>>
>>
>> Especially if there's some locality of occurrence, since analyze
>> samples pages, not rows.
>
>
> But it doesn't take all rows in each sampled page. It generally takes about
> one row per page, specifically to avoid the problem you indicate. Maybe it
> is possible to trick it into taking too many (for example, if most pages are
> completely empty), but I haven't experienced that as being a problem.
Still, I remember a discussion where it was clear there was a bias
towards sampling rows from the same page (or nearby), resulting in
this particular problem.
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