Re: efficiency of wildcards at both ends

AFAIK, wildcards at both ends are not optimized at all, unless you use
some sort of specialized index (may be Gist or FullText).
Until 9.1 there is no such "Index Scan" feature, that would help (very
little).
Other databases (like MS SQL Server) solve this kind of query by
executing an Index Scan, then merge join with rest of the query.

This is all I know about LIKE optimization in PostgreSQL:

LIKE 'str%' -> optimized by normal indexes
LIKE '%str%' -> not optimized. You can use FullText, but then your
wildcards will have to change to something not SQL-standard compatible
solution...
LIKE '%str' -> can be optimized if you create index with column content
reversed then query reversed as well. See code below for details.

How did I optimized "%str" queries (code implemented with help from the
PgSql community):

CREATE OR REPLACE FUNCTION reverse(input character varying)
RETURNS character varying AS
$BODY$
DECLARE
result character varying = '';
i int;
BEGIN
FOR i IN 1..length(input) BY 2 LOOP
result = substr(input,i+1,1) || substr(input,i,1) || result;
END LOOP;
RETURN result;
END$BODY$
LANGUAGE plpgsql IMMUTABLE STRICT
COST 100;
create index idx on tb1 (reverse(nome));
select * from tb1 where reverse(nome) like reverse('%RICHTER');

Regards,

Edson.

Em 20/06/2012 14:10, Sam Z J escreveu:
> Hi all
>
> I'm curious how is wildcards at both ends implemented, e.g. LIKE '%str%'
> How efficient is it if that's the only search criteria against a large
> table? how much does indexing the column help and roughly how much
> more space is needed for the index?
>
> if the answers are too long, please point me to the relavant text =D
>
> thanks
>
> --
> Zhongshi (Sam) Jiang
> sammyjiang721(at)gmail(dot)com <mailto:sammyjiang721(at)gmail(dot)com>