Adding indexes to a table is slowing down query performance.

Question

I am running a query against a table which contains approx. 4 million records in it. So I created 4 indexes on the table and noticed that the performance on my query drastically decreased. I went back and began remove and creating the indexes in different combinations. It turns out that whenever two of four indexes are created the performance worsens. The strange thing about this problem is when I do an explain plan on the query the cost is greater when the performance is better and the cost is less when the performance is worse. Also Oracle only uses one out of the four indexes on the table for this query.
I'd like to try to understand what is going on with the Oracle optimizer to try to fix this problem.

Answer 1

Mark,
Below is the information you requested.
DATABASE: 10.2.0.3.0
QUERY:
select distinct object, object_access from betweb_objects
where instr(object_access,'RES\') = 0
and object_access_type = 'ADM'
and object in (select distinct object
from betweb_objects
where instr(object_access,'RES\') = 0
and object_access_type = 'NTK'
and object not like '%.%'
and substr(object_access,instr(object_access,'\')+1) in (select distinct substr(object_access,instr(object_access,'\')+1)
from betweb_objects
where object_access_type = 'NTK'
and instr(object_access,'RES\') = 0
minus
select distinct upper(id)
from uamp.ad_users
where status = 'A'))
TABLE:
BETWEB_OBJECTS
OBJECT                VARCHAR2
OBJECT_ACCESS           VARCHAR2
OBJECT_ACCESS_TYPE      VARCHAR2
INDEXES ON BETWEB_OBJECTS:
BETWEB_OBJECTS_IDX1
OBJECT
BETWEB_OBJECTS_IDX2
OBJECT_ACCESS
BETWEB_OBJECTS_IDX3
OBJECT_ACCESS_TYPE
BETWEB_OBJECTS_IDX4
OBJECT_ACCESS
OBJECT_ACCESS_TYPE
TABLE:
AD_USERS
ID                VARCHAR2
DOMAIN           VARCHAR2
FNAME           VARCHAR2
LNAME           VARCHAR2
INITIALS           VARCHAR2
TITLE           VARCHAR2
DN                VARCHAR2
COMPANY           VARCHAR2
DEPARTMENT      VARCHAR2
PHONE           VARCHAR2
MANAGER           VARCHAR2
STATUS           VARCHAR2
DISPLAY_NAME      VARCHAR2
EXPLAIN PLAN when performance is better:
SELECT STATEMENT      Rows=13,414      Time=643,641      Cost=53,636,676      Bytes=6,948,452
HASH UNIQUE           Rows=13,414      Time=643,641      Cost=53,636,676      Bytes=6,948,452
HASH JOIN           Rows=694,646,835      Time=428           Cost=35,620      Bytes=359,827,060,530
VIEW VW_NSO_1           Rows=542           Time=42           Cost=3,491           Bytes=163,684
MINUS
SORT UNIQUE           Rows=542                               Bytes=9,756
INDEX FAST FULL SCAN BETWEB_OBJECTS_IDX4 Rows=26,427      Time=40           Cost=3,302           Bytes=475,686
SORT UNIQUE           Rows=16,228                          Bytes=178,508
TABLE ACCESS FULL AD_USERS Rows=16,360      Time=2           Cost=113           Bytes=179,960
HASH JOIN                Rows=128,163,623      Time=322           Cost=26,805      Bytes=27,683,342,568
TABLE ACCESS FULL BETWEB_OBJECTS Rows=9,161           Time=154           Cost=12,805      Bytes=989,388
TABLE ACCESS FULL BETWEB_OBJECTS Rows=25,106      Time=154           Cost=12,822          Bytes=2,711,448
EXPLAIN PLAN when performance is worse:
SELECT STATEMENT      Rows=13,414      Time=22,614      Cost=1,884,484      Bytes=2,897,424
HASH UNIQUE           Rows=13,414      Time=22,614      Cost=1,884,484      Bytes=2,897,424
HASH JOIN                Rows=128,163,623      Time=322           Cost=26,805      Bytes=27,683,342,568
TABLE ACCESS FULL BETWEB_OBJECTS Rows=9,161           Time=154           Cost=12,805      Bytes=989,388
TABLE ACCESS FULL BETWEB_OBJECTS Rows=25,106      Time=154           Cost=12,822      Bytes=2,711,448
MINUS
SORT UNIQUE NOSORT      Rows=209           Time=40           Cost=3,305           Bytes=3,762
INDEX FAST FULL SCAN BETWEB_OBJECTS_IDX4 Rows=264           Time=40           Cost=3,304           Bytes=4,752
SORT UNIQUE NOSORT      Rows=164           Time=2           Cost=115           Bytes=1,804
TABLE ACCESS FULL AD_USERS Rows=164           Time=2           Cost=114           Bytes=1,804