Query performance with %

Question

Hi,
I have a system running on 11gr2 windows 64bit windows with Oracle text.
90 % of the data is in Hebrew and the rest is in English.
we have a great performance while running Q with % in the end of the word like : word%
when we issue a search with the % in the beginning like : %word , of the word the performance are extremely bad.
__I have create a test case :__
-- CREATE TABLE
CREATE TABLE news (pkey NUMBER,lang VARCHAR2 (2), short_content CLOB);
-- INSERT DATA
insert into news values (myseq.nextval,'iw','&1');
-- The next step is to configure the MULTI_LEXER
BEGIN
-- hebrew
ctx_ddl.create_preference ('hebrew_lexer', 'basic_lexer');
--english
ctx_ddl.create_preference('english_lexer','basic_lexer');
ctx_ddl.set_attribute('english_lexer','index_themes','yes');
ctx_ddl.set_attribute('english_lexer','theme_language','english');
END;
-- CREATE THE MULTI_LEXER
--Create the multi-lexer preference:
BEGIN
ctx_ddl.create_preference('global_lexer', 'multi_lexer');
END;
-- make the hebrew lexer the default using CTX_DDL.ADD_SUB_LEXER:
BEGIN
ctx_ddl.add_sub_lexer('global_lexer','default','hebrew_lexer');
END;
--add the English  language with CTX_DDL.ADD_SUB_LEXER procedure.
BEGIN
ctx_ddl.add_sub_lexer('global_lexer','english','english_lexer','eng');
END;
-- create the wordlist
begin
-- ADD WORDLIST
-- exec ctx_ddl.drop_preference ('my_wordlist');
ctx_ddl.create_preference ('my_wordlist','basic_wordlist');
ctx_ddl.set_attribute     ('my_wordlist', 'stemmer','auto');
ctx_ddl.set_attribute ('my_wordlist','SUBSTRING_INDEX', 'YES');
end;
--CREATE THE INDEX 
--drop index search_news
create index search_news
on news (short_content)
indextype is ctxsys.context
parameters
('lexer          global_lexer
     language column lang
     wordlist     my_wordlist')
still the performance are bad.
I know I am missing here somthing.
I appropriate any help

Answer 1

That's expected. Internally Oracle Text has a list of words (the $I table) on which there is an index (the $X index).
If you use a leading wildcard, then the $X index cannot be used and Oracle Text has to do a full-table scan of the $I table.
If you MUST have leading wildcards, you should use the SUBSTRING_INDEX wordlist preference when creating the index. That creates an extra ($P) table which allows Oracle Text to resolve leading wildcards without resorting to a full table scan.
Be warned that your index creation will take considerably longer and use a lot more space with this option in place. Many customers prefer to disallow leading wildcards from their search interface.