Searching CLOB column of XML documents with leading wildcard - Performance

Question

Hi, our table has a text indexed CLOB column of XML documents and when performing a search with a leading wild card, we never retrieve any results.
The query looks like this:
select id from <table> where contains(columnname, '(%12345)') > 0;
I cant even generate an explain plan from this query. I killed it after 39 minutes.
If the query changes to:
select id from <table> where contains(columnname, '(12345%)') > 0;
I get an explain plan immediately with a cost=2 and when I execute the query, I get results in less than a second.
I'd appreciate any thoughts of what I should check or what the problem might be.
Thanks! Doug

Answer 1

Can you provide a script that reproduces the case. I am unable to reproduce the problem with just some small sample data, as shown below. That means that there is nothing wrong with the syntax, but you may be having problems due to the size of your data or other parameters that have not been mentioned.
SCOTT@10gXE> CREATE TABLE your_table (id NUMBER, columnname CLOB)
  2  /
Table created.
SCOTT@10gXE> insert into your_table
  2  select 1, dbms_xmlgen.getxml
  3             ('select deptno, dname,
  4                   cursor (select empno, ename
  5                        from      emp
  6                        where  emp.deptno = dept.deptno ) employee
  7               from   dept
  8               where  deptno = 10')
  9  from   dual
10  /
1 row created.
SCOTT@10gXE> SELECT * FROM your_table
  2  /
        ID COLUMNNAME
         1 <?xml version="1.0"?>
           <ROWSET>
            <ROW>
             <DEPTNO>10</DEPTNO>
             <DNAME>ACCOUNTING</DNAME>
             <EMPLOYEE>
              <EMPLOYEE_ROW>
               <EMPNO>7782</EMPNO>
               <ENAME>CLARK</ENAME>
              </EMPLOYEE_ROW>
              <EMPLOYEE_ROW>
               <EMPNO>7839</EMPNO>
               <ENAME>KING</ENAME>
              </EMPLOYEE_ROW>
              <EMPLOYEE_ROW>
               <EMPNO>7934</EMPNO>
               <ENAME>MILLER</ENAME>
              </EMPLOYEE_ROW>
             </EMPLOYEE>
            </ROW>
           </ROWSET>
SCOTT@10gXE> CREATE INDEX your_idx ON your_table (columnname)
  2  INDEXTYPE IS CTXSYS.CONTEXT
  3  /
Index created.
SCOTT@10gXE> EXEC DBMS_STATS.GATHER_TABLE_STATS ('SCOTT', 'YOUR_TABLE')
PL/SQL procedure successfully completed.
SCOTT@10gXE> SET AUTOTRACE ON EXPLAIN
SCOTT@10gXE> select id from your_table where contains (columnname, '(%839)') > 0
  2  /
        ID
         1
Execution Plan
Plan hash value: 2832585188
| Id  | Operation                   | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |            |     1 |   888 |     0   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| YOUR_TABLE |     1 |   888 |     0   (0)| 00:00:01 |
|*  2 |   DOMAIN INDEX              | YOUR_IDX   |       |       |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("CTXSYS"."CONTAINS"("COLUMNNAME",'(%839)')>0)
SCOTT@10gXE> SET AUTOTRACE OFF