CTXCAT index problem: SUBSTRING_INDEX parameter doesn't work

Question

Hi,
I need a help with ctxcat index on Oracle 10g. I need to improve searching using CTXCAT index. All is working quite well except SUBSTRING_INDEX parameter. I thought that following was necessary and sufficient:
EXEC ctx_ddl.create_preference('KEVFT_WL', 'BASIC_WORDLIST');
EXEC ctx_ddl.set_attribute('KEVFT_WL', 'substring_index', 'YES');
EXEC ctx_ddl.set_attribute('KEVFT_WL', 'prefix_index', 'YES');
EXEC ctx_ddl.set_attribute('KEVFT_WL', 'prefix_min_length', 1);
EXEC ctx_ddl.set_attribute('KEVFT_WL', 'prefix_max_length', 6);
This BASIC_WORDLIST preference set is used for index creation. But the problem is, that when I try to search using query:
SELECT * FROM TMP_FULLTEXT
WHERE CATSEARCH(fulltext, '%abc', null)> 0;
then I get no rows in results. Despite the fact that row with the text 'aabc' exists in the table.
Can anyone tell me what the problem is?
Thank you,
Julius Chrobak

Answer 1

With a catsearch query operator, the wildcard is an asterisk (*), not a percent sign (%) as with a contains query operator. However, even with the correct wildcard, left truncation is not supported with ctxcat. However, if you use a query template, then you can use the context grammar with the % wildcard for context grammar and left truncation is supported. Please see the demonstration below that reproduces the problem, then corrects it.
SCOTT@10gXE> SELECT * FROM v$version
  2  /
BANNER
Oracle Database 10g Express Edition Release 10.2.0.1.0 - Product
PL/SQL Release 10.2.0.1.0 - Production
CORE     10.2.0.1.0     Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
SCOTT@10gXE> CREATE TABLE tmp_fulltext (fulltext VARCHAR2 (30))
  2  /
Table created.
SCOTT@10gXE> INSERT ALL
  2  INTO tmp_fulltext VALUES ('aabc')
  3  INTO tmp_fulltext VALUES ('abcd')
  4  SELECT * FROM DUAL
  5  /
2 rows created.
SCOTT@10gXE> BEGIN
  2    ctx_ddl.create_preference ('KEVFT_WL', 'BASIC_WORDLIST');
  3    ctx_ddl.set_attribute      ('KEVFT_WL', 'substring_index',   'YES');
  4    ctx_ddl.set_attribute      ('KEVFT_WL', 'prefix_index',        'YES');
  5    ctx_ddl.set_attribute      ('KEVFT_WL', 'prefix_min_length', 1);
  6    ctx_ddl.set_attribute      ('KEVFT_WL', 'prefix_max_length', 6);
  7  END;
  8  /
PL/SQL procedure successfully completed.
SCOTT@10gXE> CREATE INDEX fulltext_idx ON tmp_fulltext (fulltext)
  2  INDEXTYPE IS CTXSYS.CTXCAT
  3  PARAMETERS ('WORDLIST KEVFT_WL')
  4  /
Index created.
SCOTT@10gXE> -- reproduction of original problem using wrong wildcard for catsearch:
SCOTT@10gXE> SELECT * FROM TMP_FULLTEXT
  2  WHERE CATSEARCH (fulltext, '%abc', null) > 0
  3  /
no rows selected
SCOTT@10gXE> SELECT * FROM TMP_FULLTEXT
  2  WHERE CATSEARCH (fulltext, 'abc%', null) > 0
  3  /
no rows selected
SCOTT@10gXE> -- left truncation not supported, even with correct wildcard:
SCOTT@10gXE> SELECT * FROM TMP_FULLTEXT
  2  WHERE CATSEARCH (fulltext, '*abc', null) > 0
  3  /
no rows selected
SCOTT@10gXE> SELECT * FROM TMP_FULLTEXT
  2  WHERE CATSEARCH (fulltext, 'abc*', null) > 0
  3  /
FULLTEXT
abcd
SCOTT@10gXE> -- query using template with context grammar:
SCOTT@10gXE> SELECT * FROM TMP_FULLTEXT
  2  WHERE CATSEARCH
  3            (fulltext,
  4             '<query>
  5             <textquery grammar="context">
  6               %abc
  7             </textquery>
  8              <query>',
  9             null) > 0
10  /
FULLTEXT
aabc
SCOTT@10gXE>