Issue While executing the Query for Pagination using ROWNUM with like

Question

Issue While executing the Query for Pagination using ROWNUM with like.
Database is Oracle11G.
Oracle Database Table contains 8-9 lakh records
1) SQL equal (=)
SELECT /*+ FIRST_ROWS(n) */ ROWNUM RNUM, A.* FROM LINE A
WHERE A.REFERENCE = 'KMF22600920'
Execution Time:- 0.00869245 seconds
Returns 2 resultsets
2) SQL like (one %)
SELECT /*+ FIRST_ROWS(n) */ ROWNUM RNUM, A.* FROM LINE A
WHERE A.REFERENCE = 'KMF22600920%'
Execution Time:- 0.01094301 seconds
Returns 2 resultsets
3) SQL like (two%)
SELECT /*+ FIRST_ROWS(n) */ ROWNUM RNUM, A.* FROM LINE A
WHERE A.REFERENCE like '%KMF22600920%'
Execution Time:- 6.43989658 seconds
Returns 2 resultsets
In Pagination, we are using Modified version of SQL Query 3) with ROWNUM as mentioned below :-
4) SELECT * FROM (
SELECT /*+ FIRST_ROWS(n) */ ROWNUM RNUM, A.* FROM LINE A
WHERE REFERENCE like '%KMF22600920%' AND ROWNUM <= 20 ) WHERE RNUM > 0
Execution Time:- Infinite
ResultSets:- No as execution time is infinite
a) Instead of like if we use = in the above query it is returning the 2 resultsets (execution time 0.02699282 seconds)
b) Instead of two % in the above query, if use one example REFERENCE like 'KMF22600920%' it is returning the 2 resultsets (execution time 0.03313019 seconds)
Issue:- When using two % in like in the above query i.e. REFERENCE like '%KMF22600920%' AND ROWNUM <= 20 ) , it is going to infinite.
Could you please let us know what is the issue with two % used in like and rownum
5) Modified version of Option1 query (move out the RNUM condition AND RNUM <= 20)
SELECT * FROM (
SELECT /*+ FIRST_ROWS(n) */ ROWNUM RNUM, A.* FROM LINE A
WHERE REFERENCE like '%KMF22600920%' ) WHERE RNUM > 0 AND RNUM <= 20
Execution Time:- 7.41368914 seconds
Returns 2 resultsets
Is the above query is best optimized query which should be used for the Pagination or still can improve on this ?

Answer 1

This would be easier to diagnose if there was an explain plan posted for the 'good' and 'bad' queries. Generally speaking using '%' on both sides precludes the use of any indexes.