Thread: seq scan on a table whereas an index exist

seq scan on a table whereas an index exist

From

Nicolas GIMMILLARO

Date:

07 March 2001, 16:28:03

I don't understand the query plan used to resolve this query :
select * from web_access where date between '01/01/2001' and
'06/03/2001'

The web_access table as this structure :

         Table "web_access"
 Attribute |     Type     | Modifier
-----------+--------------+----------
 source    | varchar(128) | not null
 date      | date         |
 heure     | time         |
 action    | varchar(128) |
 code1     | integer      |
 code2     | integer      |
 util      | text         |
 refer     | varchar(128) |
 browser   | varchar(20)  |
 brorel    | varchar(10)  |
 os        | varchar(20)  |
 osrel     | varchar(10)  |
Indices: web_access_date,
         web_access_source

Index "web_access_date"
 Attribute | Type
-----------+------
 date      | date
btree

A classical select on table web_access uses a seq scan :
-----------------------------------------------------------------------------------------------------------

explain select * from web_access where date between '01/01/2001' and
'06/03/2001';
NOTICE:  QUERY PLAN:

Seq Scan on web_access  (cost=0.00..35380.36 rows=117694 width=116)
-----------------------------------------------------------------------------------------------------------

The same query with 2 sub-queries uses an index scan :
-----------------------------------------------------------------------------------------------------------

explain select * from web_access where date between (select
'01/01/2001'::date)  and (select '06/03/2001'::date);
NOTICE:  QUERY PLAN:

Index Scan using web_access_date on web_access  (cost=0.00..26858.67
rows=7908 width=116)
  InitPlan
    ->  Result  (cost=0.00..0.00 rows=0 width=0)
    ->  Result  (cost=0.00..0.00 rows=0 width=0)
-----------------------------------------------------------------------------------------------------------



What's wrong with my "select * from web_access where date between
'01/01/2001' and '06/03/2001' "  query ?

Nicolas GIMMILLARO

Re: seq scan on a table whereas an index exist

From

Tom Lane

Date:

08 March 2001, 00:07:19

Nicolas GIMMILLARO <Nicolas.GIMMILLARO@wmi.fr> writes:
> What's wrong with my "select * from web_access where date between
> '01/01/2001' and '06/03/2001' "  query ?

Nothing.  The planner is estimating that this query will retrieve a
sufficiently large fraction of the rows in the table that a seqscan
will be cheaper than an indexscan.  That estimate might or might not
be correct, but it's operating as designed.

The form with the subselects confuses the planner enough that it falls
back on default selectivity estimates, which do lead it to choose an
indexscan.

            regards, tom lane