Re: Optimizing Query

Wow. I can't believe the difference. It didn't take too long. I'll set up a
script in my etc/cron.weekly to run it... would there be any benefit to
doing a vacuum analyze nightly?

Justin Long

At 11:10 PM 3/5/2001 -0500, you wrote:
>Justin Long <justinlong(at)strategicnetwork(dot)org> writes:
> > Ok, now I have another question... it doesn't seem to be accessing the
> index.
>
> > explain select k.kbid,k.title from knowledge k , kbwords w0 , kbwords w1
> > WHERE ((k.kbid=w0.kbid and w0.wordid=42743) AND (k.kbid=w1.kbid and
> > w1.wordid=85369))
>
> > NOTICE: QUERY PLAN:
>
> > Merge Join (cost=32339.30..35496.97 rows=19262538 width=24)
> > -> Merge Join (cost=16530.24..16668.77 rows=233274 width=20)
> > -> Sort (cost=15809.06..15809.06 rows=8257 width=4)
> > -> Seq Scan on kbwords w1 (cost=0.00..15271.85 rows=8257
> > width=4)
> > -> Sort (cost=721.18..721.18 rows=2825 width=16)
> > -> Seq Scan on knowledge k (cost=0.00..559.25 rows=2825
> > width=16)
> > -> Sort (cost=15809.06..15809.06 rows=8257 width=4)
> > -> Seq Scan on kbwords w0 (cost=0.00..15271.85 rows=8257
> width=4)
>
> > Note the sequential scans... there is a wordindex where w0.wordid=42743...
> > why isn't it doing an indexscan? wouldn't that be more efficient?
>
>It probably thinks not, because the estimated number of hits (8257) is
>so high. That estimate is currently driven by the frequency of the most
>common value in the column (mainly because that's the only stat we have
>:-(). I am guessing that you have a few very common words, which are
>skewing the stats for kbwords and causing it not to pick an indexscan.
>
>Does your setup have a notion of "stop words" that shouldn't be indexed,
>like "a", "an", "the", etc? Perhaps you need to add such a feature, or
>throw in a few more stopwords if you already have 'em.
>
> regards, tom lane
>
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____________________________________________________________________
Justin Long Network for Strategic Missions
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