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

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