Subject: Re: TB-sized databases
From: Simon Riggs <simon@2ndquadrant.com>
To: Robert Treat <xzilla@users.sourceforge.net>
Cc: pgsql-performance@postgresql.org, Tom Lane <tgl@sss.pgh.pa.us>,
	Gregory Stark <stark@enterprisedb.com>, Csaba Nagy <nagy@ecircle-ag.com>,
	Bill Moran <wmoran@collaborativefusion.com>
In-Reply-To: <200712051507.22745.xzilla@users.sourceforge.net>
References: <4544e0330711260904g45d893a8ub1731e245aa8fc84@mail.gmail.com>
	<13851.1196351131@sss.pgh.pa.us>
	<1196352866.4246.1304.camel@ebony.site>
	<200712051507.22745.xzilla@users.sourceforge.net>
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Date: Thu, 06 Dec 2007 09:38:16 +0000
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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