| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Euler Taveira de Oliveira <euler(at)timbira(dot)com> |
| Cc: | Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>, Greg Stark <gsstark(at)mit(dot)edu>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Scara Maccai <m_lists(at)yahoo(dot)it>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Query progress indication - an implementation |
| Date: | 2009-07-02 18:27:28 |
| Message-ID: | 603c8f070907021127y22dbd504n324ef20e620800d9@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Jul 2, 2009 at 12:48 PM, Euler Taveira de
Oliveira<euler(at)timbira(dot)com> wrote:
> I know that it didn't solve the estimation problem but ... IMHO the
> [under|over]estimation should be treated by an external tool (autoexplain?).
> So when we enable the query progress and some node reports a difference
> between estimated and real more than x%, log the plan. Doing it, we will be
> helping DBAs to investigate the bad plans.
Keep in mind that it is frequently the case that the estimates are
substantially off but the plan still works OK. I just put a dirty
hack into one of my apps to improve the selectivity estimates by a
factor of 200, but they're still off by a factor of 5. Even when they
were off by 1000x the bad plan happened only intermittently. You
notice the cases where the estimates are off and it makes for a bad
plan, but there are lots of other cases where the estimates are off
but the plan is still OK.
...Robert
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