Use of Index although a great number of rows is returned....

Question

Hi ,
I have created the following table:
create table all_obj select * from all_objects;
Then i created some indexes:
create index idx_obj on all_obj (object_name)
create index idx_owner on all_obj (owner)
Then i issued the sql statement:
SQL> select count(object_name),owner from all_obj group by owner;
COUNT(OBJECT_NAME) OWNER
               719 MDSYS
               266 SCOTT_BI
               266 INFO_BI
                 2 TSMSYS
               118 DMSYS
20146 PUBLIC
                 6 OUTLN
               171 CTXSYS
               612 OLAPSYS
               400 SYSTEM
               168 EXFSYS
43 SCOTT
                32 DBSNMP
              1638 ORDSYS
                 5 ORDPLUGINS
               866 SYSMAN
               284 XDB
20993 SYS
               264 INFORMATICS
               212 WMSYS
COUNT(OBJECT_NAME) OWNER
                 8 SI_INFORMTN_SCHEMA
21 rows selectedand...:
SQL> analyze table all_obj compute statistics;
Table analyzedAt last , i issued the following pair of sql statements:
SQL> select object_name,owner from scott.all_obj where owner='SCOTT'
  2  /
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Plan hash value: 2272571446
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| T
ime     |
|   0 | SELECT STATEMENT            |           |  2249 | 65221 |    68   (0)| 0
0:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| ALL_OBJ   |  2249 | 65221 |    68   (0)| 0
0:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX_OWNER |  2249 |       |     6   (0)| 0
0:00:01 |
Predicate Information (identified by operation id):
   2 - access("OWNER"='SCOTT')
ÓôáôéóôéêÜ
          0  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
       1692  bytes sent via SQL*Net to client
        407  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         43  rows processedand
SQL> select object_name,owner from scott.all_obj where owner='SYS';
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Plan hash value: 2272571446
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| T
ime     |
|   0 | SELECT STATEMENT            |           |  2249 | 65221 |    68   (0)| 0
0:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| ALL_OBJ   |  2249 | 65221 |    68   (0)| 0
0:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX_OWNER |  2249 |       |     6   (0)| 0
0:00:01 |
Predicate Information (identified by operation id):
   2 - access("OWNER"='SYS')
ÓôáôéóôéêÜ
          0  recursive calls
          0  db block gets
       3345  consistent gets
          0  physical reads
          0  redo size
     801069  bytes sent via SQL*Net to client
      15774  bytes received via SQL*Net from client
       1401  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      20993  rows processed
The question is: why the second query uses the index since a great number of records is returned......????
I use Oracle10g.v.2
Thanks...
Sim

Answer 1

You were right.....
using:
SQL> EXEC DBMS_STATS.gather_schema_stats(ownname => 'SCOTT');
the optimizer does follow the 'waited' access path....for the 'PUBLIC' records.....
It seems that using the analyze table statement does not permit to gather all the necessary statistics.....(that's why it is depreciated)...!!!!
Thanks.....
Sim