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Re: Query tuning

From: "Dave Dutcher" <dave(at)tridecap(dot)com>
To: "'Subbiah, Stalin'" <SSubbiah(at)netopia(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Query tuning
Date: 2006-08-23 23:19:59
Message-ID: 02a501c6c70a$a7ff61d0$8300a8c0@tridecap.com (view raw or flat)
Thread:
Lists: pgsql-performance
It seems to me that what would work best is an index scan backward on the
eventtime index.  I don't see why that wouldn't work for you, maybe the
planner is just esitmating the seq scan and sort is faster for some reason.
What does EXPLAIN say if you use a small limit and offset like 10?  Or what
does EXPLAIN say if you first run "set enable_seqscan=false;"  (If you get
the same plan, then I wouldn't bother running EXPLAIN ANALYZE, but if you
get a different plan I would run EXPLAIN ANALYZE to see if the new plan is
any faster.)



> -----Original Message-----
> From: pgsql-performance-owner(at)postgresql(dot)org 
> [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of 
> Subbiah, Stalin
> Sent: Wednesday, August 23, 2006 1:03 PM
> To: Chris
> Cc: pgsql-performance(at)postgresql(dot)org
> Subject: Re: [PERFORM] Query tuning
> 
> 
> I get the same plan after running vacuum analyze. Nope, I don't have
> index on objdomainid, objid and userdomainid. Only eventime has it.
> 
> -----Original Message-----
> From: Chris [mailto:dmagick(at)gmail(dot)com] 
> Sent: Tuesday, August 22, 2006 8:06 PM
> To: Subbiah, Stalin
> Cc: pgsql-performance(at)postgresql(dot)org
> Subject: Re: [PERFORM] Query tuning
> 
> Subbiah, Stalin wrote:
> > Actually these servers will be upgraded to 8.1.4 in couple 
> of months.
> 
> even so, you could get some bad data in there.
> http://www.postgresql.org/docs/8.0/static/release.html . Go 
> through the
> old release notes and you'll find various race conditions, 
> crashes etc.
> 
> > Here you go with explain analyze.
> > 
> > # explain analyze SELECT *
> > FROM EVENTLOG
> > WHERE EVENTTIME>'07/23/06 16:00:00' AND  
> EVENTTIME<'08/22/06 16:00:00'
> 
> > AND  (OBJDOMAINID='tzRh39d0d91luNGT1weIUjLvFIcA' 
> >         OR OBJID='tzRh39d0d91luNGT1weIUjLvFIcA' 
> >         OR USERDOMAINID='tzRh39d0d91luNGT1weIUjLvFIcA')
> > ORDER BY EVENTTIME DESC, SEQUENCENUM DESC LIMIT 500 OFFSET 500;
> >  
> > QUERY PLAN
> > 
> > 
> ----------------------------------------------------------------------
> > --
> > 
> ----------------------------------------------------------------------
> > --
> > 
> ----------------------------------------------------------------------
> > --
> > 
> ----------------------------------------------------------------------
> > --
> > -------------------------------------------------------------
> >  Limit  (cost=15583110.14..15583111.39 rows=500 width=327) (actual
> > time=427771.568..427772.904 rows=500 loops=1)
> >    ->  Sort  (cost=15583108.89..15618188.88 rows=14031998 
> width=327) 
> > (actual time=427770.504..427771.894 rows=1000 loops=1)
> >          Sort Key: eventtime, sequencenum
> >          ->  Seq Scan on eventlog  (cost=0.00..2334535.17 
> > rows=14031998
> > width=327) (actual time=10.370..190038.764 rows=7699388 loops=1)
> >                Filter: ((eventtime > '2006-07-23 
> 16:00:00'::timestamp 
> > without time zone) AND (eventtime < '2006-08-22 
> 16:00:00'::timestamp 
> > without time zone) AND (((objdomainid)::text =
> > 'tzRh39d0d91luNGT1weIUjLvFIcA'::text) OR ((objid)::text =
> > 'tzRh39d0d91luNGT1weIUjLvFIcA'::text) OR ((userdomainid)::text =
> > 'tzRh39d0d91luNGT1weIUjLvFIcA'::text)))
> >  Total runtime: 437884.134 ms
> > (6 rows)
> 
> If you analyze the table then run this again what plan does 
> it come back
> with?
> 
> I can't read explain output properly but I suspect (and I'm 
> sure I'll be
> corrected if need be) that the sort step is way out of whack and so is
> the seq scan because the stats aren't up to date enough.
> 
> Do you have an index on objdomainid, objid and userdomainid (one index
> per field) ? I wonder if that will help much.
> 
> --
> Postgresql & php tutorials
> http://www.designmagick.com/
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