Question

I’d like to index queries like x like '%abc%'

If I have a table like the following

create table t
(
  data varchar(100)
);

I want to create an index to be able to do the following efficiently:

select * from t where contains('%abc%');

And this:

select * from t where contains('abc%');

I also want this table to be updated live.

How do I create such an index? (I have a feeling I need a ctxcat index, but I’m confused about what options I need to give it)

I’m using Oracle 10g.

Answer 1

I would use this (set you min and max length to appropiate values)

BEGIN
    ctx_ddl.create_preference  ('FT_WL', 'BASIC_WORDLIST');
    ctx_ddl.set_attribute      ('FT_WL', 'substring_index',   'YES');
    ctx_ddl.set_attribute      ('FT_WL', 'prefix_index',      'YES');
    ctx_ddl.set_attribute      ('FT_WL', 'prefix_min_length', 1);
    ctx_ddl.set_attribute      ('FT_WL', 'prefix_max_length', 6);
  END;

CREATE INDEX fulltext_idx ON tmp_fulltext (fulltext)
 INDEXTYPE IS CTXSYS.CTXCAT
 PARAMETERS ('WORDLIST FT_WL')

The parameters are explained here Oracle Text Reference

and see this question on how to manage the refresh and how the index may not be quicker than a full scan with high cardinality data:

PL/SQL Performance Tuning for LIKE '%…%' Wildcard Queries