Just ocurred to me that would be
possible to create some sort of "hybrid" solution...
create index idx1 on tb1 (nome);
create index idx2 on tb1 (reverse(nome));
select * from tb1
where nome like 'CARLOS%' or reverse(nome) like
reverse('%CARLOS')
Should return same results as
select * from tb1
where nome like '%CARLOS%'
I supposed that this hybrid solution will be optmized by the
indexes (but at what cost?).
Can some PostgreSQL expert tell if this assumption is right? Would
be possible to have PostgreSQL doing that automatically? Something
like
create index idx1 on tb1 (nome) with options (optimize wildcards);
select * from tb1 where nome like '%CARLOS%';
and then this get expanded as the example above? Then, what
happens with the following query:
select * from tb1 where nome like '%CARLOS%ERICKSSON%';
?
Edson
Em 20/06/2012 14:28, Edson Richter escreveu:
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
