In a SQL query whihc has join, How to reduce Multiple instance of a table

Question

in a SQL query which has join, How to reduce Multiple instance of a table
Here is an example: I am using Oracle 9i
is there a way to reduce no.of Person instances from the following query? or can I optimize this query further?
TABLES:
mail_table
mail_id, from_person_id, to_person_id, cc_person_id, subject, body
person_table
person_id, name, email
QUERY:
SELECT p_from.name from, p_to.name to, p_cc.name cc, subject
FROM mail, person p_from, person p_to, person p_cc
WHERE from_person_id = p_from.person_id
AND to_person_id = p_to.person_id
AND cc_person_id = p_cc.person_id
Thnanks in advance,
Babu.

Answer 1

SQL> select * from mail;
        ID          F          T         CC
         1          1          2          3
SQL> select * from person;
       PID NAME
         1 a
         2 b
         3 c
--Query with only ne Instance of PERSON Table
SQL> select m.id,max(decode(m.f,p.pid,p.name)) frm_name,
  2         max(decode(m.t,p.pid,p.name)) to_name,
  3         max(decode(m.cc,p.pid,p.name)) cc_name
  4  from mail m,person p
  5  where m.f = p.pid
  6  or m.t = p.pid
  7  or m.cc = p.pid
  8  group by m.id;
        ID FRM_NAME   TO_NAME    CC_NAME
         1 a          b          c
--Expalin plan for "One instance" Query
SQL> explain plan for
  2  select m.id,max(decode(m.f,p.pid,p.name)) frm_name,
  3         max(decode(m.t,p.pid,p.name)) to_name,
  4         max(decode(m.cc,p.pid,p.name)) cc_name
  5  from mail m,person p
  6  where m.f = p.pid
  7  or m.t = p.pid
  8  or m.cc = p.pid
  9  group by m.id;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 902563036
| Id  | Operation           | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT    |        |     3 |   216 |     7  (15)| 00:00:01 |
|   1 |  HASH GROUP BY      |        |     3 |   216 |     7  (15)| 00:00:01 |
|   2 |   NESTED LOOPS      |        |     3 |   216 |     6   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| MAIL   |     1 |    52 |     3   (0)| 00:00:01 |
|*  4 |    TABLE ACCESS FULL| PERSON |     3 |    60 |     3   (0)| 00:00:01 |
PLAN_TABLE_OUTPUT
Predicate Information (identified by operation id):
   4 - filter("M"."F"="P"."PID" OR "M"."T"="P"."PID" OR
              "M"."CC"="P"."PID")
Note
   - dynamic sampling used for this statement
--Explain plan for "Normal" query
SQL> explain plan for
  2  select m.id,pf.name fname,pt.name tname,pcc.name ccname
  3  from mail m,person pf,person pt,person pcc
  4  where m.f = pf.pid
  5  and m.t = pt.pid
  6  and m.cc = pcc.pid;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 4145845855
| Id  | Operation            | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT     |        |     1 |   112 |    14  (15)| 00:00:01 |
|*  1 |  HASH JOIN           |        |     1 |   112 |    14  (15)| 00:00:01 |
|*  2 |   HASH JOIN          |        |     1 |    92 |    10  (10)| 00:00:01 |
|*  3 |    HASH JOIN         |        |     1 |    72 |     7  (15)| 00:00:01 |
|   4 |     TABLE ACCESS FULL| MAIL   |     1 |    52 |     3   (0)| 00:00:01 |
|   5 |     TABLE ACCESS FULL| PERSON |     3 |    60 |     3   (0)| 00:00:01 |
PLAN_TABLE_OUTPUT
|   6 |    TABLE ACCESS FULL | PERSON |     3 |    60 |     3   (0)| 00:00:01 |
|   7 |   TABLE ACCESS FULL  | PERSON |     3 |    60 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - access("M"."CC"="PCC"."PID")
   2 - access("M"."T"="PT"."PID")
   3 - access("M"."F"="PF"."PID")
PLAN_TABLE_OUTPUT
Note
   - dynamic sampling used for this statement
25 rows selected.
Message was edited by:
        jeneesh
No indexes created...