Re: trouble with (lack of) indexing

On Thu, May 09, 2002 at 07:38:06PM -0400, Tom Lane wrote:
> =?iso-8859-1?Q?S=F8ren?= Boll Overgaard <postgres@fork.dk> writes:
> >  explain SELECT ht.id,hq.ip,hq.id FROM
> >  hostsqueue as hq,hoststests as ht WHERE  ht.hostsqueue_id=hq.id;
> > NOTICE:  QUERY PLAN:
>
> > Hash Join  (cost=776.05..2904.36 rows=84 width=24)
> >   ->  Seq Scan on hoststests ht  (cost=0.00..2126.84 rows=84 width=8)
> >   ->  Hash  (cost=776.04..776.04 rows=4 width=16)
> >         ->  Seq Scan on hostsqueue hq  (cost=0.00..776.04 rows=4 width=16)
>
>
> If there's only 84 rows it's hardly going to matter which plan we choose
> ;-).  Please show us the results from the production database, not the
> toy-sized tables.
>
> If you have 7.2 then EXPLAIN ANALYZE is much more useful to show than
> plain EXPLAIN.  Also, you could try doing "SET enable_seqscan = off"
> and see how the plan changes.

I missed the outputs of the explain analyze queries in the previous mail:
explain analyze SELECT ht.id,hq.ip,hq.id FROM hostsqueue as hq,hoststests as ht WHERE  ht.hostsqueue_id=hq.id;
NOTICE:  QUERY PLAN:

Hash Join  (cost=75331.89..127823.94 rows=160293 width=24) (actual time=122572.70..146683.59 rows=142807 loops=1)
  ->  Seq Scan on hoststests ht  (cost=0.00..49686.93 rows=160293 width=8) (actual time=170.79..21255.68 rows=142807
loops=1)
  ->  Hash  (cost=75243.51..75243.51 rows=35351 width=16) (actual time=122401.10..122401.10 rows=0 loops=1)
        ->  Seq Scan on hostsqueue hq  (cost=0.00..75243.51 rows=35351 width=16) (actual time=518.88..122024.50
rows=28610loops=1) 
Total runtime: 147598.79 msec

EXPLAIN

And after setting the enable_seqscan to off, I got this:

=> SET enable_seqscan = off;
SET VARIABLE
=> explain analyze SELECT ht.id,hq.ip,hq.id FROM hostsqueue as hq,hoststests as ht WHERE  ht.hostsqueue_id=hq.id;
NOTICE:  QUERY PLAN:

Merge Join  (cost=0.00..772324.63 rows=160293 width=24) (actual time=8319.78..116061.18 rows=141645 loops=1)
  ->  Index Scan using hostsqueue_pkey on hostsqueue hq  (cost=0.00..137603.50 rows=35351 width=16) (actual
time=34.90..90831.97rows=28623 loops=1) 
  ->  Index Scan using idx_htsts_hq on hoststests ht  (cost=0.00..632228.35 rows=160293 width=8) (actual
time=18.41..23343.75rows=141645 loops=1) 
Total runtime: 116562.03 msec

EXPLAIN

Actually, I was rather hoping for a large gain in performance, but maybe the
"total runtimes" are not actally representative of performance of the actual
selects?

--
Søren O.                                       ,''`.
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public key: finger boll <at> db.debian.org    `. `'
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