Query takes 5 min when using Indexes and 1 Second without Indexes !!

Question

Hi,
We have been using indexes on all tables until recently when we faced problems with queries like the one below:
SELECT a.std_id FROM students a, student_degree b, student_course c, course d
WHERE b.std_id = a.std_id
AND c.std_id = a.std_id
AND d.crn = c.crn
AND b.in_progress = 'Y'
AND b.major_code = 'ABTC'
AND a.program_code = 'DP'
AND a.level_code = 'S2'
AND a.campus_code = '05'
AND a.termcode = '200702';
This query takes more than 5 minutes to return a result, but as soon as we remove indexes on the columns termcode and campus_code,it shows result in 1 second.
What could be the problem ?, Is there an attribute that need to be set when creating these indexes ?
Thanks in advance
Madani

Answer 1

Thank you Karthik for your reply.Here are the explain plan report (as shown on Oracle 9i Enterprise Manager)
*1-Explain plan without Indexes:*
Execution Steps:
Step # Step Name
11 SELECT STATEMENT
10 MERGE JOIN
7 SORT [JOIN]
6 NESTED LOOPS
4 NESTED LOOPS
1 ERMS.STUDENT_DEGREE TABLE ACCESS [FULL]
3 ERMS.STUDENTS TABLE ACCESS [BY INDEX ROWID]
2 ERMS.SYS_C006642 INDEX [UNIQUE SCAN]
5 ERMS.SYS_C007065 INDEX [RANGE SCAN]
9 SORT [JOIN]
8 ERMS.COURSE TABLE ACCESS [FULL]
Step # Description
1 This plan step retrieves all rows from table STUDENT_DEGREE.
2 This plan step retrieves a single ROWID from the B*-tree index SYS_C006642.
3 This plan step retrieves rows from table STUDENTS through ROWID(s) returned by an index.
4 This plan step joins two sets of rows by iterating over the driving, or outer, row set (the first child of the join) and, for each row, carrying out the steps of the inner row set (the second child). Corresponding pairs of rows are tested against the join condition specified in the query's WHERE clause.
5 This plan step retrieves one or more ROWIDs in ascending order by scanning the B*-tree index SYS_C007065.
6 This plan step joins two sets of rows by iterating over the driving, or outer, row set (the first child of the join) and, for each row, carrying out the steps of the inner row set (the second child). Corresponding pairs of rows are tested against the join condition specified in the query's WHERE clause.
7 This plan step accepts a row set (its only child) and sorts it in preparation for a merge-join operation.
8 This plan step retrieves all rows from table COURSE.
9 This plan step accepts a row set (its only child) and sorts it in preparation for a merge-join operation.
10 This plan step accepts two sets of rows sorted on the join key. By walking both sets of rows in the order of the join key, every distinct pair of rows satisfying the join condition in the WHERE clause is found through a single pass of the row sets.
11 This plan step designates this statement as a SELECT statement.
*2-Explain plan with Indexes:* (I added index on column campus_code which is a varchar2 column)
Execution Steps:
Step # Step Name
11 SELECT STATEMENT
10 MERGE JOIN
7 SORT [JOIN]
6 NESTED LOOPS
4 NESTED LOOPS
1 ERMS.COURSE TABLE ACCESS [FULL]
3 ERMS.STUDENTS TABLE ACCESS [BY INDEX ROWID]
2 ERMS.INDEX_STUDENTS_CAMPUS_CODE INDEX [RANGE SCAN]
5 ERMS.SYS_C007065 INDEX [RANGE SCAN]
9 SORT [JOIN]
8 ERMS.STUDENT_DEGREE TABLE ACCESS [FULL]
Step # Description
1 This plan step retrieves all rows from table COURSE.
2 This plan step retrieves one or more ROWIDs in ascending order by scanning the B*-tree index INDEX_STUDENTS_CAMPUS_CODE.
3 This plan step retrieves rows from table STUDENTS through ROWID(s) returned by an index.
4 This plan step joins two sets of rows by iterating over the driving, or outer, row set (the first child of the join) and, for each row, carrying out the steps of the inner row set (the second child). Corresponding pairs of rows are tested against the join condition specified in the query's WHERE clause.
5 This plan step retrieves one or more ROWIDs in ascending order by scanning the B*-tree index SYS_C007065.
6 This plan step joins two sets of rows by iterating over the driving, or outer, row set (the first child of the join) and, for each row, carrying out the steps of the inner row set (the second child). Corresponding pairs of rows are tested against the join condition specified in the query's WHERE clause.
7 This plan step accepts a row set (its only child) and sorts it in preparation for a merge-join operation.
8 This plan step retrieves all rows from table STUDENT_DEGREE.
9 This plan step accepts a row set (its only child) and sorts it in preparation for a merge-join operation.
10 This plan step accepts two sets of rows sorted on the join key. By walking both sets of rows in the order of the join key, every distinct pair of rows satisfying the join condition in the WHERE clause is found through a single pass of the row sets.
11 This plan step designates this statement as a SELECT statement.