Re: what is the cause that scan type is showing as 'seq scan' after - Mailing list pgsql-admin

Gaetano, thanks.

My db has only one table (about 29,000 records), so I thought leaving
postgreqsql.conf at its defaults would be OK: the params you mention are
commented out, so they must be at their defaults, tho I can't tell what the
defaults are.
Are there any docs that talk specificially about how to set these params and
what the defaults are (the official docs don't say  much)?

Thanks.


--- Gaetano Mendola <mendola@bigfoot.com> wrote:
> Joseph Lemm wrote:
> > IN RELATION TO THIS POST:
> >
> > On Sun, Jan 26, 2003 at 06:31:50PM +0530, shreedhar wrote:
> >
> >>Hi All,
> >>
> >>Before indexing query plan was showing cost as 40.00, after indexing query
> >>plan again showing as 'seq scan' and cost as 3060.55.
> >>The field which i indexed is primary key to this table.
> >>May i know
> >>1) what is the cause that scan type is showing as 'seq scan' after indexing
> >>also
> >>2) why it is showing cost as high value compare to previous.
> >
> >
> > TO WHICH ROSS REPLIED:
> >
> >
> >>You trimmed out the other parts of the EXPLAIN, so I'm just guessing,
> >>but that cost seems suspiciously round: I'm guessing that you haven't
> >>run VACUUM ANALYZE at all. One thing indexing does is update the 'number
> >>of tuples' statistic. See the archives for why sequential scans still
> >>show up (short answer: index scans aren't free, so at some point, it's
> >>cheaper to scan the entire table than to scan both the index and the
> >>subset of the table returned)
> >
> >
> >
> >
> > OK, so then what is the explanation for this:
> >
> >                Table "public.post"
> >  Column |            Type             | Modifiers
> > --------+-----------------------------+-----------
> >  id     | integer                     |
> >  author | character varying(80)       |
> >  text   | text                        |
> >  hidden | boolean                     |
> >  date   | timestamp without time zone |
> >  host   | character varying(80)       |
> > Indexes: idx_post_id unique btree (id),
> >          post_author_index btree (author)
> >
> >
> > VACUUM ANALYZE;
> > VACUUM
> >
> > EXPLAIN ANALYZE select host from post where author='George';
> >                                                QUERY PLAN
> >
> >
>
--------------------------------------------------------------------------------------------------------
> >  Seq Scan on post  (cost=0.00..2869.30 rows=1768 width=27) (actual
> > time=0.23..520.65 rows=1774 loops=1)
> >    Filter: (author = 'George'::character varying)
> >  Total runtime: 525.77 msec
> > (3 rows)
> >
> > So the optimizer decided it's less costly to do a sequential scan here than
> use
> > the index, right?
> >
> >
> > Now:
> >
> > SET ENABLE_SEQSCAN=OFF;
> >
> > EXPLAIN ANALYZE select host from post where author='George';
> >                                                             QUERY PLAN
> >
>

-----------------------------------------------------------------------------------------------------------------------------------
> >  Index Scan using post_author_index on post  (cost=0.00..5253.63 rows=1768
> > width=27) (actual time=28.92..210.25 rows=1774 loops=1)
> >    Index Cond: (author = 'George'::character varying)
> >  Total runtime: 215.00 msec
> > (3 rows)
> >
> >
> > So if I force an index scan, I get much better performance (215 vs 525
> msec).
> > Does this mean that the optimizer screwed up when it recommended a
> sequential
> > scan?
>
> No this mean that you are instructing your optimizer in a wrong way.
>
>
> Show us your configuration file and in particular these parameters:
>
> effective_cache_size
> random_page_cost
> cpu_tuple_cost
> cpu_index_tuple_cost
> cpu_operator_cost
>
> I use these value, that are good enough for a medium HW:
>
> effective_cache_size = 20000
> random_page_cost = 2.5
> cpu_tuple_cost = 0.005
> cpu_index_tuple_cost = 0.0005
> cpu_operator_cost = 0.0025
>
>
> Regards
> Gaetano Mendola
>
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> ---------------------------(end of broadcast)---------------------------
> TIP 8: explain analyze is your friend


=====
J.

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