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Thread: Index not used ! Why?

Index not used ! Why?

From
"Gabor Csuri"
Date:
Hi All,
my database server has very high load in this morning.
I've found the problem. One of my index was not used so far!
it's interesting:

addb=> \d banners                                     Table "banners"Attribute  |           Type           |
         Modifier                      
 
------------+--------------------------+----------------------------------------------------b_no       | integer
         | not null default nextval('banners_b_no_seq'::text)usr_no     | integer                  | b_ext      |
charactervarying(10)    | b_link     | character varying(100)   | b_from     | date                     | b_to       |
date                    | b_lastview | timestamp with time zone | default now()b_maxview  | integer                  |
b_curview | integer                  | default 0b_maxclick | integer                  | b_curclick | integer
     | default 0b_weight   | integer                  | default 1b_curwg    | double precision         | default
0b_active  | boolean                  | default 'f'::boollast_upd   | timestamp with time zone | default now()upd_usr
| integer                  | b_name     | character varying(40)    | 
 
Indices: b_usr_no_idx,        banners_b_no_key

addb=> EXPLAIN SELECT b_link FROM banners WHERE b_no = 3;
NOTICE:  QUERY PLAN:

Seq Scan on banners  (cost=0.00..1.57 rows=1 width=12)

EXPLAIN
addb=> DROP INDEX banners_b_no_key;
DROP
addb=> CREATE INDEX banners_b_no_key ON banners (b_no);
CREATE
addb=> EXPLAIN SELECT b_link FROM banners WHERE b_no = 3;
NOTICE:  QUERY PLAN:

Index Scan using banners_b_no_key on banners  (cost=0.00..4.43 rows=1 width=12)

EXPLAIN
addb=> 

Why index wasn't used ?
postgresql-7.1.2, redhat 7.0, kernel:2.2.19

Thanks, Gabor

Re: Index not used ! Why?

From
"Gabor Csuri"
Date:
> Hello!
> It needs some help by the command
> VACUUM [VERBOSE] ANALYZE table;
> to choose the ideal query strategy.

How can I choose better query strategy than  ...WHERE key_field = x; ?

Regards, Gabor.

Re: Index not used ! Why? + Little graphical client ...

From
"Nicolas Verger"
Date:
>  my database server has very high load in this morning.
> I've found the problem. One of my index was not used so far!
> it's interesting:
> ...
> addb=> CREATE INDEX banners_b_no_key ON banners (b_no);
> CREATE
> addb=> EXPLAIN SELECT b_link FROM banners WHERE b_no = 3;
> NOTICE:  QUERY PLAN:
>
> Index Scan using banners_b_no_key on banners  (cost=0.00..4.43
> rows=1 width=12)
>
> EXPLAIN
> addb=>
>
> Why index wasn't used ?
> postgresql-7.1.2, redhat 7.0, kernel:2.2.19

Try to create a unique index :
CREATE UNIQUE INDEX banners_b_no_key ON banners (b_no);
or specify a primary key :
ALTER TABLE banners ADD CONSTRAINT pk_banners PRIMARY KEY (b_no);

then ANALYZE your table ....

-- Nicolas --

We ( me and my teammate ) try to create a little graphical client for
PostgreSQL in Java. If someone want to try it :
http://pgInhaler.ifrance.com. It's an alpha version with lots of bugs... Try
it and send us your feedback to pginhaler@ifrance.com... Thanx...