Wildcard search queries

Question

Oracle 10g 10.2.0.4
Linux
we are having some queries where users are using wildcard for searching data in varchar2 fields. when they use like 'text%" the results are returning fast as it's able to use index. But queries are slow when they use like '%text%' or even like '%text'.
Kindly suggest.
check below for explain plan.
SQL> select * from t_invoice_header where invoice like 'REIM113%';
Execution Plan
Plan hash value: 1419012284
| Id  | Operation                   | Name                  | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |                       |    75 | 26175 |    68   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T_INVOICE_HEADER      |    75 | 26175 |    68   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | T_INVOICE_HEADER_IDX2 |    75 |       |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("INVOICE" LIKE 'REIM113%')
       filter("INVOICE" LIKE 'REIM113%')
SQL>  select * from t_invoice_header where invoice like '%REIM113%';
Execution Plan
Plan hash value: 717214792
| Id  | Operation         | Name             | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |                  |  2620K|   872M|   418K  (2)| 01:37:42 |
|*  1 |  TABLE ACCESS FULL| T_INVOICE_HEADER |  2620K|   872M|   418K  (2)| 01:37:42 |
Predicate Information (identified by operation id):
   1 - filter("INVOICE" LIKE '%REIM113%')
SQL>

Answer 1

Think of how an index would be used. An index on INVOICE will be sorted according to the INVOICE, i.e. alphabetically. If you don't specify the first few characters of the INVOICE value, Oracle can't search the index. And it has no idea how selective the condition will be (if you've specified the first few characters, Oracle can get a reasonable guess on selectivity). So Oracle would either have to do a full scan on the index and then do an unknown series of lookups or it would have to do a table scan. The optimizer expects that the table scan is the more efficient plan-- without more information, I have no reason to expect that the optimizer is incorrect.
Depending on what the INVOICE column contains, you could potentially create an Oracle Text index and use a CONTAINS operator rather than using LIKE to search for strings within the string. But that assumes that the invoice data can be parsed reasonably in building the Text index.
Justin