question: db2 LUW V8 UNION ALL  with table function month() have bad
query performance

Env:
db2 LUW V8 + FP14

Problem :
We have history data from 2005/01/01 ~ 2007/05/xx  in single  big
table,
we try  separate this big table into twelve tables and create a view
to join these 12 tables with UNION ALL .
after query the view, we get bad query performance than query single
big table.

follow our scenario  :

create table sales_01(
sales_date  date not null,
prod_id integer,
city_id  integer,
channel_id integer,
revenue decimal(20,2) ) ;

create table sales_02 like sales_01 ;
create table sales_03 like sales_01 ;
create table sales like sales_01 ;
create table sales_05 like sales_01 ;
create table sales_06 like sales_01 ;
create table sales_07 like sales_01 ;
create table sales_08 like sales_01 ;
create table sales_09 like sales_01 ;
create table sales_10 like sales_01 ;
create table sales_11 like sales_01 ;
create table sales_12 like sales_01 ;

alter table sales_01 add constraint date_01
check(month(sales_date)=1);
alter table sales_02 add constraint date_02
check(month(sales_date)=2);
alter table sales_03 add constraint date_03
check(month(sales_date)=3);
alter table sales_04 add constraint date_04
check(month(sales_date)=4);
alter table sales_05 add constraint date_05
check(month(sales_date)=5);
alter table sales_06 add constraint date_06
check(month(sales_date)=6);
alter table sales_07 add constraint date_07
check(month(sales_date)=7);
alter table sales_08 add constraint date_08
check(month(sales_date)=8);
alter table sales_09 add constraint date_09
check(month(sales_date)=9);
alter table sales_10 add constraint date_10
check(month(sales_date)=10);
alter table sales_11 add constraint date_11
check(month(sales_date)=11);
alter table sales_12 add constraint date_12
check(month(sales_date)=12);

create view all_sales as
(
select * from sales_01
union all
select * from sales_02
union all
select * from sales
union all
select * from sales_05
union all
select * from sales_06
union all
select * from sales_07
union all
select * from sales_08
union all
select * from sales_09
union all
select * from sales_10
union all
select * from sales_11
union all
select * from sales_12
) ;

test select sql
select * from all_sales_04 where sales_date  > '2007-10-15' ;

explain command:
db2expln -d sample  -f sel.sql -t -g -z ;

explain  output :

SQL Statement:

 select *
 from all_sales_04
 where sales_date > '2007-10-15'

Section Code Page = 850

Estimated Cost = 142.257706
Estimated Cardinality = 267.666626

(

UNION

UNION

UNION

UNION

UNION

UNION

UNION

UNION

UNION

UNION

)
Return Data to Application

End of section

our currently solution is re-create view similar follow :
create view all_sales_05 as
(
select * from sales_01
where sales_date between '2007-01-01' and '2007-01-31' or
sales_date between '2006-01-01' and '2006-01-31' or
sales_date between '2005-01-01' and '2005-01-31'
union all
select * from sales_02
where sales_date between '2007-02-01' and '2007-02-28'
union all
select * from sales_03
where sales_date between '2007-03-01' and '2007-03-31'
union all
select * from sales_04
where sales_date between '2007-04-01' and '2007-04-30'
union all
select * from sales_05
where sales_date between '2007-05-01' and '2007-05-31'
union all
select * from sales_06
where sales_date between '2007-06-01' and '2007-06-30'
union all
select * from sales_07
where sales_date between '2007-07-01' and '2007-07-31'
union all
select * from sales_08
where sales_date between '2007-08-01' and '2007-08-31'
union all
select * from sales_09
where sales_date between '2007-09-01' and '2007-09-30'
union all
select * from sales_10
where sales_date between '2007-10-01' and '2007-10-31'
union all
select * from sales_11
where sales_date between '2007-11-01' and '2007-11-30'
union all
select * from sales_12
where sales_date between '2007-12-01' and '2007-12-31'

) ;

new exlpain output:
 select *
 from all_sales_05
 where sales_date > '2007-10-15'

Section Code Page = 850

Estimated Cost = 38.844070
Estimated Cardinality = 21.900002

(

UNION

UNION

)
Return Data to Application

End of section

but our data is continue growing and this view is need to maintain
every year for DBA,
did anyone know other simple  solution can maintain this kind data ?
Thanks