Re: [GENERAL] Generic search - Mailing list pgsql-general

Hello Jerome,

venerdì, 4 dicembre 98, you wrote:

jd> Hi,
jd> As it seems
jd> Postgres doesn't use index when the where clause contains "like" operator.
jd> It only uses it in case of "=" operator.
jd> for example :
test=>> create table essai (nom varchar(50));
jd> CREATE
test=>> create index essai_nom on essai (nom);
jd> CREATE
test=>> \d essai

jd> Table    = essai
jd> +----------------------------------+----------------------------------+-----
jd> --+
jd> |              Field               |              Type                |
jd> Length|
jd> +----------------------------------+----------------------------------+-----
jd> --+
jd> | nom                              | varchar()                        |
jd> 50 |
jd> +----------------------------------+----------------------------------+-----
jd> --+
test=>> \d essai_nom

jd> Table    = essai_nom
jd> +----------------------------------+----------------------------------+-----
jd> --+
jd> |              Field               |              Type                |
jd> Length|
jd> +----------------------------------+----------------------------------+-----
jd> --+
jd> | nom                              | varchar()                        |
jd> -4 |
jd> +----------------------------------+----------------------------------+-----
jd> --+
test=>> explain select * from essai where nom='abc';
jd> NOTICE:  QUERY PLAN:

jd> Index Scan using essai_nom on essai  (cost=0.00 size=0 width=12)

jd> EXPLAIN
test=>> explain select * from essai where nom like 'a%';
jd> NOTICE:  QUERY PLAN:

jd> Seq Scan on essai  (cost=0.00 size=0 width=12)

jd> EXPLAIN

jd> Postgres 6.4 Linux 2.0.34.

jd> So I'm looking for a way to get quite good performances with generic
jd> search.
jd> Thank you for advices.
jd> Jerome.

How many records do you have in this table ?
If you have only few records PostgreSQL doesn't use index because it
is faster to read in sequential mode.
Try to insert a lot of records. It works, look:

hygea=> explain select * from comuni where nome = 'A%';
NOTICE:  QUERY PLAN:
Index Scan using nome_comune_idx on comuni  (cost=2.05 size=2 width=84)
^^^^^
EXPLAIN
hygea=> explain select * from comuni where nome > 'A%';
NOTICE:  QUERY PLAN:
Index Scan using nome_comune_idx on comuni  (cost=215.03 size=2781 width=84)
^^^^^
EXPLAIN
hygea=> explain select * from comuni where nome <> 'A%';
NOTICE:  QUERY PLAN:
Seq Scan on comuni  (cost=427.29 size=8342 width=84)
EXPLAIN
hygea=> explain select * from comuni where nome < 'A%';
NOTICE:  QUERY PLAN:
Index Scan using nome_comune_idx on comuni  (cost=215.03 size=2781 width=84)
^^^^^
EXPLAIN
hygea=> explain select * from comuni where nome like '%A%';
NOTICE:  QUERY PLAN:
Seq Scan on comuni  (cost=427.29 size=2 width=84)
^^^
Pg uses index with LIKE only if the first char is not a special char.

-Jose'-