How to dynamically change the table name inside a view

Question

Hi All,
create table t_auto_feeds
id number,
table_name vachar2(100));
insert into t_auto_feeds values(1,'T_FEED_POSITIONS');
insert into t_auto_feeds values(2,'T_KAP_MTM');
assume there are 100 records on id=1 in T_FEED_POSITIONS and 100 records of id=2 in T_KAP_MTM (id 1 is present only in t_feed_positions & id 2 is present only in t_kap_mtm)
i need to create a view such that it needs to give the count of records based on the id
create or replace view aa_view as
select count(*), id from t_feed_position group by id
union
select count(*), id from t_kap_mtm group by id;
I am getting a proper result when i query the view like select * from aa_view where id=1 but will the other union query seems to be a overhead?. I am having 10 such tables configured for different id in t_auto_feeds.so do i need to put 10 unions or is there a better way to handle this stuff.
I know i can use a PLSQL block and dynamically build view structure, the problem is the view itself is configured in a table , so i cant write a proc for this.. can this aa_view be modified such that it queries only the table(using the t_auto_feeds ) which matches the id rather than the entire list.
Kindly help me in this regard and let me know in case u need any further information from my side.

Answer 1

>
so do i need to put 10 unions or is there a better way to handle this stuff.
>
Yes - you need 10 unions
Yes - there is a better way so that only one of the 10 queries does anything.
You can use SYS_CONTEXT to control the query selection.
Here is an example of using SYS_CONTEXT. Try this code in the SCOTT schema.
create or replace context VIEW_CTX using SET_VIEW_FLAG;
create or replace procedure SET_VIEW_FLAG ( p_table_name in varchar2 default 'EMP')
  as
  begin
      dbms_session.set_context( 'VIEW_CTX', 'TABLE_NAME', upper(p_table_name));
  end;
select * from emp where 'EMP' = sys_context( 'VIEW_CTX', 'TABLE_NAME' );
select * from emp1 where 'EMP1' = sys_context( 'VIEW_CTX', 'TABLE_NAME' );
select * from emp2 where 'EMP2' =  sys_context( 'VIEW_CTX', 'TABLE_NAME' )
exec set_view_flag( p_table_name => 'EMP' );
exec set_view_flag( p_table_name => 'EMP1' );
exec set_view_flag( p_table_name => 'EMP2');
SELECT sys_context( 'VIEW_CTX', 'TABLE_NAME' ) FROM DUAL
CREATE VIEW THREE_TABLE_EMP_VIEW AS
select * from emp where 'EMP' = sys_context( 'VIEW_CTX', 'TABLE_NAME' )
union all
select * from emp1 where 'EMP1' = sys_context( 'VIEW_CTX', 'TABLE_NAME' )
union all
select * from emp2 where 'EMP2' =  sys_context( 'VIEW_CTX', 'TABLE_NAME' )
SELECT * FROM THREE_TABLE_EMP_VIEWNote that you set the context for the table you want. This doesn't have to be a table name it could just be flag value
-- use the emp table
exec set_view_flag( p_table_name => 'EMP' );
-- ue the emp1 table
exec set_view_flag( p_table_name => 'EMP1' );
-- use the emp2 table
exec set_view_flag( p_table_name => 'EMP2');