Oracle - Questions on indexing

Question

If i have order by/group by clauses in my SQL then is it a good idea to crete indexes on the tables?
if in a SQL, there are two tables. both are being joined. One of which is being used in LIKE clause filter. then should i create index on the other table?
if i have a SQL with three tables. table A is getting joined to table B and C. then should i create two indexes (one for each join) or one index only?

Answer 1

Here is a test case that proves you wrong.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> drop table t
  2  /
Table dropped.
SQL> create table t
  2  as
  3  select * from all_objects
  4  /
Table created.
SQL> create index t_idx on t(object_id)
  2  /
Index created.
SQL> exec dbms_stats.gather_table_stats(user, 'T', cascade=>true)
PL/SQL procedure successfully completed.
SQL>  explain plan
  2   for
  3   select owner         ,
  4     object_name   ,
  5     subobject_name,
  6     data_object_id,
  7     object_type   ,
  8     created       ,
  9     last_ddl_time ,
10     timestamp
11     from t
12    order by object_id
13  /
Explained.
SQL> select * from table(dbms_xplan.display)
  2  /
PLAN_TABLE_OUTPUT
Plan hash value: 3778778741
| Id  | Operation                   | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |       |   116K|  9339K|  2542   (1)| 00:00:31 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T     |   116K|  9339K|  2542   (1)| 00:00:31 |
|   2 |   INDEX FULL SCAN           | T_IDX |   116K|       |   264   (2)| 00:00:04 |
-------------------------------------------------------------------------------------The selection criteria between Sort or index scan depends on lots of other factors and not as that of what you have mentioned. May be this could be of your interest.
http://www.dba-oracle.com/oracle_news/2004_12_3_webb_oracle_sorting_index_access.htm
Thanks,
Karthick.