Also there is huge impact on performance.

The purpose of mview is to have response time decrease.

From what is said, give an example of possible mview.

select * from mytable

create  materialized view m_mytable enable query rewrite
as select * from mytable ;

select * from mytable

select * from m_mytable where till_date<sysdate

orcl> create materialized view mv1 as select * from hr.regions;

Materialized view created.

orcl> select object_name,object_type from user_objects where object_name='MV1';

OBJECT_NAME          OBJECT_TYPE
-------------------- -------------------
MV1                  TABLE
MV1                  MATERIALIZED VIEW

orcl> select segment_name,segment_type from user_segments where segment_name='MV1';

SEGMENT_NAME         SEGMENT_TYPE
-------------------- ------------------
MV1                  TABLE

orcl>

SQL> set autotrace traceonly statistics
SQL> select t_code,count(*) from pm group by t_code order by 1;


Statistics
----------------------------------------------------------
        505  recursive calls
          0  db block gets
       1340  consistent gets
       1114  physical reads
          0  redo size
        589  bytes sent via SQL*Net to client
        469  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          5  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> create materialized view m_pm enable query rewrite
  2  as
  3  select t_code,count(*) from pm group by t_code order by 1;

Materialized view created.

SQL>select object_name,object_type from user_objects where object_name='M_PM'

SQL> set autotrace off
SQL> /

OBJECT_NAME
--------------------------------------------------------------------------------OBJECT_TYPE
-------------------
M_PM
TABLE

M_PM
MATERIALIZED VIEW


SQL> select segment_name,segment_type from user_segments where segment_name='M_PM';

SEGMENT_NAME
--------------------------------------------------------------------------------SEGMENT_TYPE
------------------
M_PM
TABLE


SQL> alter system flush shared_pool;

System altered.

SQL> set autotrace traceonly statistics
SQL> select t_code,count(*) from pm group by t_code order by 1;


Statistics
----------------------------------------------------------
       7555  recursive calls
          0  db block gets
       1532  consistent gets
         17  physical reads
        124  redo size
        589  bytes sent via SQL*Net to client
        469  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
         84  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> set autotrace traceonly
SQL> /


Execution Plan
----------------------------------------------------------
Plan hash value: 4232873480

--------------------------------------------------------------------------------------

| Id  | Operation                     | Name | Rows  | Bytes | Cost (%CPU)| Time     |

--------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT              |      |     1 |    17 |     3  (34)| 00:00:01 |

|   1 |  SORT ORDER BY                |      |     1 |    17 |     3  (34)| 00:00:01 |

|   2 |   MAT_VIEW REWRITE ACCESS FULL| M_PM |     1 |    17 |     2   (0)| 00:00:01 |

--------------------------------------------------------------------------------------


Note
-----
   - dynamic sampling used for this statement


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

SQL> ed
Wrote file afiedt.buf

  1* select /*+ no_rewrite */ t_code,count(*) from pm group by t_code order by 1
SQL> /


Execution Plan
----------------------------------------------------------
Plan hash value: 3673671942

-------------------------------------------------------------------------------------

| Id  | Operation            | Name         | Rows  | Bytes | Cost (%CPU)| Time
    |

-------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT     |              |     1 |     5 |    11   (0)| 00:00:01 |

|   1 |  SORT GROUP BY NOSORT|              |     1 |     5 |    11   (0)| 00:00:01 |

|   2 |   INDEX FULL SCAN    | PM_PK |   232K|  1133K|    11   (0)| 00:00:01 |

-------------------------------------------------------------------------------------



Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       1111  consistent gets
          0  physical reads
          0  redo size
        589  bytes sent via SQL*Net to client
        469  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed