Re: TB-sized databases - Mailing list pgsql-performance

From Simon Riggs
Subject Re: TB-sized databases
Date
Msg-id 1196933896.4255.340.camel@ebony.site
Whole thread Raw
In response to Re: TB-sized databases  (Robert Treat <xzilla@users.sourceforge.net>)
Responses Re: TB-sized databases
Re: TB-sized databases
List pgsql-performance
Robert,

On Wed, 2007-12-05 at 15:07 -0500, Robert Treat wrote:

> If the whole performance of your system depends upon indexed access, then
> maybe you need a database that gives you a way to force index access at the
> query level?

That sounds like a request for hints, which is OT here, ISTM.

The issue is that if somebody issues a "large query" then it will be a
problem whichever plan the query takes. Forcing index scans can make a
plan more expensive than a seq scan in many cases.

> > e.g. An 80GB table has 8 million blocks in it.
> > - So putting a statement_cost limit = 1 million would allow some fairly
> > large queries but prevent anything that did a SeqScan (or worse).
> > - Setting it 10 million is going to prevent things like sorting the
> > whole table without a LIMIT
> > - Setting it at 100 million is going to prevent unconstrained product
> > joins etc..
>
> I think you're completly overlooking the effect of disk latency has on query
> times.  We run queries all the time that can vary from 4 hours to 12 hours in
> time based solely on the amount of concurrent load on the system, even though
> they always plan with the same cost.

Not at all. If we had statement_cost_limit then it would be applied
after planning and before execution begins. The limit would be based
upon the planner's estimate, not the likely actual execution time.

So yes a query may vary in execution time by a large factor as you
suggest, and it would be difficult to set the proposed parameter
accurately. However, the same is also true of statement_timeout, which
we currently support, so I don't see this point as an blocker.

Which leaves us at the burning question: Would you use such a facility,
or would the difficulty in setting it exactly prevent you from using it
for real?

--
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com


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