Case Insensitive Indexes

Question

In relation to switching on case insensitive queries using
alter session set NLS_COMP=LINGUISTIC;Can anyone answer the following?
>
Yes, it works.... but I can't for the life of me figure out how to build a linguistic index that the LIKE clause will actually use. Building an index thus, for example:
create index bin_ai on names(NLSSORT("NAME",'nls_sort=''BINARY_AI'''));
makes an index which does get used to good effect by queries such as
select name from names where name = 'Johny Jacobson';
but not by
select name from names where name like 'Johny%';
Hence, in a real-world test with 100,000 records, the LIKE query runs about 100 times slower than the '=' query (3 sec compared to 0.03 sec). Not very scalable. Is there a way to speed this up??
Also is it possible to set session variables such as nls_comp on a database/schema/user level?

Answer 1

Hi,
select name from names where name like 'Johny%';Performance when using the SQL "like" clause can be tricky because the wildcard "%" operator can invalidate the index. For example a last_name index would be OK with a "like 'SMI%'" query, but unusable with "like '%SMI%'.
One obscure trick for indexing queries "like '%SON'" is to create a REVERSE index and them programmatically reverse the like clause to read "like 'NOS%'", effectively indexing on the other side of the text.
You might want to look at Oracle*text indexes, if your database has low DML:
http://www.dba-oracle.com/oracle_tips_like_sql_index.htm
If you are 10gr2:
Oracle 10g release 2 has now introduced a case insensitive search method for SQL that avoids index invalidation and unnecessary full-table scans. You can also employ Oracle text indexes to remove full-table scans when using the LIKE operator. Prior to Oracle10g release 2 case insensitive queries required special planning:
1 - Transform data in the query to make it case insensitive (note that this can invalidate indexes without a function-based index):
create index upper_full_name on customer ( upper(full_name));
select full_name from customer
where upper(full_name) = 'DON BURLESON';
2 - Use a trigger to transform the data to make it case insensitive (or store the data with the to_lower or to_upper BIF.
3 - Use Alter session commands:
alter session set NLS_COMP=ANSI;
alter session set NLS_SORT=GENERIC_BASELETTER;
select * from customer where full_name = 'Don Burleson'
Hope this helps. . .
Don Burleson
Oracle Press author