Prefix Index and Substing Index not working

Question

Dear all,
I'm trying to set the following preference to create my index:
begin
ctx_ddl.create_preference('substring_pref', 'BASIC_WORDLIST');
ctx_ddl.set_attribute('substring_pref','PREFIX_INDEX','TRUE');
ctx_ddl.set_attribute('substring_pref','PREFIX_MIN_LENGTH',3);
ctx_ddl.set_attribute('substring_pref','PREFIX_MAX_LENGTH',15);
ctx_ddl.set_attribute('substring_pref','SUBSTRING_INDEX','TRUE');
end;
create index artikel_test_idx
on artikel_test (art_content)
indextype is ctxsys.context
parameters ( 'WORDLIST substring_pref' )
The index is created sucessfully.
Here is the data I have in my table:
ART_INHOUD ART_TITEL ART_CONTENT
1 Titel1 prion alzheimer
2 Titel2 prion word1 alzheimer
3 Titel3 prion word2 word3 alzheimer
4 Titel4 prion
5 Titel5 alzheimer
6 Titel6 alzheimer prion
7 Titel7 alzheimer word1 prion
8 Titel8 something else
9 Titel9 prion alzheimer prion alzheimer
10 Title10 i am theree
11 Title11 contract initiation work
12 Title12 oracle university else
12 rows selected.
I test the substring feature by searching for "alz". It should returns me all lines that contains "alzheimer". Am I right? But the result show nothing:
select art_titel, score(1) as score
from artikel_test
where contains (art_content, 'alz', 1) > 0
no rows selected
If I search for the entire word, it works:
select art_titel, score(1) as score
from artikel_test
where contains (art_content, 'alzheimer', 1) > 0;
ART_TITEL SCORE
Titel9 7
Titel7 4
Titel6 4
Titel5 4
Titel3 4
Titel2 4
Titel1 4
7 rows selected.
I look at the table DR$ARTIKEL_TEST_IDX$I and all works are in it:
TOKEN_TEXT
ALZ
ALZH
ALZHE
ALZHEI
ALZHEIM
ALZHEIME
ALZHEIMER
ALZHEIMER
AM
CON
CONT
CONTR
CONTRA
CONTRAC
CONTRACT
CONTRACT
Why there is no result when I search for 'alz'?
Regards,
Yanick
Message was edited by:
yankee75

Answer 1

The prefix_index and substring_index do not make the substrings searchable as individual words. They just speed up wildcard queries. If you search for "alt" you will only get rows that have the word "alt" in it. If you search for "alt%" you will get all rows that contain words that begin with "alt". Here are some quotes from the Oracle Text Reference, which is part of the Oracle online documentation:
"substring_index      Specify TRUE for Oracle Text to create a substring index. A substring index improves left-truncated and double-truncated wildcard queries such as %ing or %benz%. Default is FALSE."
"prefix_index      Specify TRUE to enable prefix indexing. Prefix indexing improves performance for right truncated wildcard searches such as TO%. Defaults to FALSE."