I am able to get join elimination to work for simple cases such as one-to-one relations, but not for slightly more complicated scenarios. Ultimately I want to try anchor modelling, but first I need to find a way around this problem. I'm using Oracle 12c Enterprise Edition Release 12.1.0.2.0.
DDL for my test case:
drop view product_5nf;
drop table product_color cascade constraints;
drop table product_price cascade constraints;
drop table product cascade constraints;
create table product(
product_id number not null
,constraint product_pk primary key(product_id)
);
create table product_color(
product_id number not null references product
,color varchar2(10) not null
,constraint product_color_pk primary key(product_id)
);
create table product_price(
product_id number not null references product
,from_date date not null
,price number not null
,constraint product_price_pk primary key(product_id, from_date)
);
Some example data:
insert into product values(1);
insert into product values(2);
insert into product values(3);
insert into product values(4);
insert into product_color values(1, 'Red');
insert into product_color values(2, 'Green');
insert into product_price values(1, date '2016-01-01', 10);
insert into product_price values(1, date '2016-02-01', 8);
insert into product_price values(1, date '2016-05-01', 5);
insert into product_price values(2, date '2016-02-01', 5);
insert into product_price values(4, date '2016-01-01', 10);
commit;
The 5NF view
This first view does not compile - it fails with ORA-01799: a column may not be outer-joined to a subquery. Unfortunately, this is how most of the historized views are defined when I'm looking at the online examples of anchor modelling...
create view product_5nf as
select p.product_id
,pc.color
,pp.price
from product p
left join product_color pc on(
pc.product_id = p.product_id
)
left join product_price pp on(
pp.product_id = p.product_id
and pp.from_date = (select max(pp2.from_date)
from product_price pp2
where pp2.product_id = pp.product_id)
);
Below is my attempt at fixing it. When using this view with a simple select of product_id, Oracle manages to eliminate product_color but not product_price.
create view product_5nf as
select product_id
,pc.color
,pp.price
from product p
left join product_color pc using(product_id)
left join (select pp1.product_id, pp1.price
from product_price pp1
where pp1.from_date = (select max(pp2.from_date)
from product_price pp2
where pp2.product_id = pp1.product_id)
)pp using(product_id);
select product_id
from product_5nf;
----------------------------------------------------------
| Id | Operation | Name | Rows |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 |
|* 1 | HASH JOIN OUTER | | 4 |
| 2 | INDEX FAST FULL SCAN| PRODUCT_PK | 4 |
| 3 | VIEW | | 3 |
| 4 | NESTED LOOPS | | 3 |
| 5 | VIEW | VW_SQ_1 | 5 |
| 6 | HASH GROUP BY | | 5 |
| 7 | INDEX FULL SCAN | PRODUCT_PRICE_PK | 5 |
|* 8 | INDEX UNIQUE SCAN | PRODUCT_PRICE_PK | 1 |
----------------------------------------------------------
The only solution I have found is to use scalar sub queries instead, like this:
create or replace view product_5nf as
select p.product_id
,pc.color
,(select pp.price
from product_price pp
where pp.product_id = p.product_id
and pp.from_date = (select max(from_date)
from product_price pp2
where pp2.product_id = pp.product_id)) as price
from product p
left join product_color pc on(
pc.product_id = p.product_id
)
select product_id
from product_5nf;
---------------------------------------------------
| Id | Operation | Name | Rows |
---------------------------------------------------
| 0 | SELECT STATEMENT | | 4 |
| 1 | INDEX FAST FULL SCAN| PRODUCT_PK | 4 |
---------------------------------------------------
Now Oracle sucessfully eliminates the product_price table. However, scalar sub queries are implemented differently than joins and they way they are executed simply doesn't allow me to get any acceptable performance in a real world scenario.
TL;DR
How can I rewrite the view product_5nf so that Oracle sucessfully eliminates both of the dependent tables?