From: "Andrus" <eetasoft@online.ee>
Subject: Re: How to force Postgres to use index on ILIKE
Date: Tue, 6 Jun 2006 12:57:31 +0300
Organization: Hub.Org Networking Services
Lines: 39
Message-ID: <e63jit$28fk$2@news.hub.org>
References: <e620mg$15jr$1@news.hub.org>
 <200606051426.56789.josh@agliodbs.com>
To: pgsql-performance@postgresql.org

>> SELECT toode, nimetus
>> FROM toode
>> WHERE toode      ILIKE  'x10%' ESCAPE '!'
>> ORDER BY UPPER(toode     ),nimetus  LIMIT 100
>>
>> runs 1 minute in first time for small table size.
>>
>> Toode field type is CHAR(20)
>
> 1) why are you using CHAR and not VARCHAR or TEXT?   CHAR will give you
> problems using an index, period.

1. I haven't seen any example where VARCHAR is better that CHAR for indexing
2. I have a lot of existing code. Changing CHAR to VARCHAR requires probably 
re-writing a lot of code, a huge work.

> 2) You can't use an index on ILIKE.

I'ts very sad. I expected that lower(toode) index can be used.


> You can, however, use an index on
> lower(field) if your query is properly phrased and if you've created an
> expression index on lower(field).

I tried by Postgres does not use index. Why ?

create index nimib2 on firma1.klient(lower(nimi) bpchar_pattern_ops);

explain analyze select nimi from firma1.klient where lower(nimi) like
'mokter%'

"Seq Scan on klient  (cost=0.00..9.79 rows=1 width=74) (actual
time=0.740..0.761 rows=1 loops=1)"
"  Filter: (lower((nimi)::text) ~~ 'mokter%'::text)"
"Total runtime: 0.877 ms"