Oracle初学者入门指南-索引是干什么用的?

SQL> select sql_text
  2  from v$sqltext a
  3  where a.hash_value = (
  4  select sql_hash_value from v$session b
  5  where b.sid='&sid'
  6  )
  7  order by piece asc
  8  /

SQL_TEXT
----------------------------------------------------------------
select * from i_cm_power t WHERE T.SJH='13911xxxxx6'

SQL> select index_name from dba_indexes where table_name=upper('i_cm_power');

INDEX_NAME
------------------------------

SQL> col segment_name for a20
SQL> select segment_name,bytes/1024/1024/1024 
     from dba_segments where segment_name=upper('i_cm_power');

SEGMENT_NAME         BYTES/1024/1024/1024
-------------------- --------------------
I_CM_POWER                  5.28173828125

SQL> create index idx_i_cm_power_sjh on i_cm_power(sjh);

Index created.

Elapsed: 00:20:50.73

SQL> col segment_name for a20
SQL> select segment_name,bytes/1024/1024 MB 
  2  from dba_segments where segment_name=upper('idx_i_cm_power_sjh');

SEGMENT_NAME                 MB
-------------------- ----------
IDX_I_CM_POWER_SJH         1360

SQL> 

SQL> select * from i_cm_power t WHERE T.SJH='13911xxxxx6';

Elapsed: 00:00:00.07

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'I_CM_POWER'
   2    1     INDEX (RANGE SCAN) OF 'IDX_I_CM_POWER_SJH' (NON-UNIQUE)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          6  consistent gets
          0  physical reads
          0  redo size
       1022  bytes sent via SQL*Net to client
        503  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          2  rows processed