Why does my query do full scan on table ...?

Question

I have a query with sub-query that should narrow down a search of triples:
DELETE FROM XPROCESS_TPL  t WHERE t.triple.rdf_s_id
IN
( SELECT r.value_id FROM mdsys.rdf_value$ r
  WHERE  r.value_type='UR'   AND  r.vname_suffix LIKE  'ProcessAggregate_Lens Fab%' )
the inner SELECT returns 122707 rows in 41.914 seconds
However, the outer query takes ~6 hours all together...
What I thought would happen was it should first do the sub-query,get all value_id's and then use index to find data in XPROCESS_TPL
Why is that not happening and how can I optimize this query?
I created index on rdf_s_id:
CREATE INDEX "SEMANTIC"."XPROCESS_SUB_IDX" ON "SEMANTIC"."XPROCESS_TPL" ("TRIPLE"."RDF_S_ID")

Answer 1

The query plan is the execution plan generated by the query optimizer. One way of viewing the query plan is using explain plan through SQL*Plus:
SQL> explain plan for
select empno, ename, dname, loc
from emp e, dept d
where e.deptno = d.deptno;
  2    3    4
Explained.
Elapsed: 00:00:00.05
SQL> select plan_table_output from table(dbms_xplan.display('plan_table',null,'basic,parallel,partition,predicate'));
PLAN_TABLE_OUTPUT
Plan hash value: 2219294842
| Id  | Operation          | Name |
|   0 | SELECT STATEMENT   |      |
|*  1 |  HASH JOIN         |      |
|   2 |   TABLE ACCESS FULL| DEPT |
|   3 |   TABLE ACCESS FULL| EMP  |
Predicate Information (identified by operation id):
   1 - access("E"."DEPTNO"="D"."DEPTNO")
15 rows selected.
Elapsed: 00:00:00.47
SQL>