Postgres Pro
Downloads
Home   >   mailing lists

Thread: "Group By " index usage

"Group By " index usage

From
sdfasdfas sdfasdfs
Date:
I have a table with this index:

 create index ARTISTS_NAME on ARTISTS (
 lower(AR_NAME)
 );

Te index is over a colum with this definition:

 AR_NAME              VARCHAR(256)         null,

I want to optimize this query:

 select * from artists where lower(ar_name) like
lower('a%') order by lower(ar_name) limit 20;

I think the planner should use the index i have. But
the result of the explain command is:

 explain analyze select * from artists where
lower(ar_name) like lower('a%') order by
lower(ar_name) limit 20;


            QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=20420.09..20420.14 rows=20 width=360)
(actual time=2094.13..2094.19 rows=20 loops=1)
   ->  Sort  (cost=20420.09..20433.52 rows=5374
width=360) (actual time=2094.13..2094.16 rows=21
loops=1)
         Sort Key: lower((ar_name)::text)
         ->  Index Scan using artists_name on artists
(cost=0.00..19567.09 rows=5374 width=360) (actual
time=0.11..1391.97 rows=59047 loops=1)
               Index Cond: ((lower((ar_name)::text) >=
'a'::text) AND (lower((ar_name)::text) < 'b'::text))
               Filter: (lower((ar_name)::text) ~~
'a%'::text)
 Total runtime: 2098.62 msec
(7 rows)

The "ORDER BY" clause is not using the index!. I don't
know why.

I have the locale configured to C, and the index works
well with the "like" operator.

¿Could you help me? I am really lost.



______________________________________________
Renovamos el Correo Yahoo!: ¡100 MB GRATIS!
Nuevos servicios, más seguridad
http://correo.yahoo.es

Re: "Group By " index usage

From
Tom Lane
Date:
sdfasdfas sdfasdfs <descuarinjado@yahoo.es> writes:
> I have a table with this index:
>  create index ARTISTS_NAME on ARTISTS (
>  lower(AR_NAME)
>  );

> Te index is over a colum with this definition:

>  AR_NAME              VARCHAR(256)         null,

> I want to optimize this query:

>  select * from artists where lower(ar_name) like
> lower('a%') order by lower(ar_name) limit 20;

> I think the planner should use the index i have.

Update to 7.4, or declare the column as TEXT instead of VARCHAR.
Older versions aren't very bright about situations involving
implicit coercions.

            regards, tom lane

Re: "Group By " index usage

From
"Alban Medici (NetCentrex)"
Date:
 Did you test with ILIKE instead of lower LIKE lower ?




-----Original Message-----
From: pgsql-performance-owner@postgresql.org
[mailto:pgsql-performance-owner@postgresql.org] On Behalf Of sdfasdfas
sdfasdfs
Sent: mercredi 24 novembre 2004 18:37
To: pgsql-performance@postgresql.org
Subject: [PERFORM] "Group By " index usage

I have a table with this index:

 create index ARTISTS_NAME on ARTISTS (
 lower(AR_NAME)
 );

Te index is over a colum with this definition:

 AR_NAME              VARCHAR(256)         null,

I want to optimize this query:

 select * from artists where lower(ar_name) like
lower('a%') order by lower(ar_name) limit 20;

I think the planner should use the index i have. But the result of the
explain command is:

 explain analyze select * from artists where
lower(ar_name) like lower('a%') order by
lower(ar_name) limit 20;


            QUERY PLAN
----------------------------------------------------------------------------
---------------------------------------------------------------------
 Limit  (cost=20420.09..20420.14 rows=20 width=360) (actual
time=2094.13..2094.19 rows=20 loops=1)
   ->  Sort  (cost=20420.09..20433.52 rows=5374
width=360) (actual time=2094.13..2094.16 rows=21
loops=1)
         Sort Key: lower((ar_name)::text)
         ->  Index Scan using artists_name on artists
(cost=0.00..19567.09 rows=5374 width=360) (actual
time=0.11..1391.97 rows=59047 loops=1)
               Index Cond: ((lower((ar_name)::text) >=
'a'::text) AND (lower((ar_name)::text) < 'b'::text))
               Filter: (lower((ar_name)::text) ~~
'a%'::text)
 Total runtime: 2098.62 msec
(7 rows)

The "ORDER BY" clause is not using the index!. I don't know why.

I have the locale configured to C, and the index works well with the "like"
operator.

¿Could you help me? I am really lost.



______________________________________________
Renovamos el Correo Yahoo!: ¡100 MB GRATIS!
Nuevos servicios, más seguridad
http://correo.yahoo.es

---------------------------(end of broadcast)---------------------------
TIP 8: explain analyze is your friend