rewrite query without DENSE_RANK - oracle

Question

I have one very slow query and try to optimize response time by using a materialized view. But one part is not compatible with General Restrictions on Fast Refresh.
How to rewrite it without DENSE_RANK?
create table t (id,object_id,log_cre_date) as
select 1,2,to_date('18/5/2010, 08:00','dd/mm/yyyy, hh:mi') from dual union all
select 2,2,to_date('18/5/2010, 10:00','dd/mm/yyyy, hh mi') from dual union all
select 3,3,to_date('18/5/2010, 11:00','dd/mm/yyyy, hh mi') from dual union all
select 4,3,to_date('18/5/2010, 12:10','dd/mm/yyyy, hh mi') from dual union all
select 5,4,to_date('18/5/2010, 12:20','dd/mm/yyyy, hh mi') from dual union all
select 6,4,to_date('18/5/2010, 11:30','dd/mm/yyyy, hh mi') from dual;
SELECT
MAX(t.id) KEEP(DENSE_RANK FIRST ORDER BY log_cre_date ASC) id,
t.object_id
FROM
t
GROUP BY
t.object_id

Answer 1

I am not sure the accepted answer is fast refreshable. Here is a query that definitely is:
SELECT max(cast(to_char(t.log_cre_date,'YYYYMMDDHH24MISS') || lpad(t.id,30,'0') as varchar2(80))) maxid,
t.object_id,
COUNT(*) cnt
FROM t
GROUP BY t.object_id;
The idea is to append the id to the log_cre_date and take the max of the concatenation. That way, you can extract the id you need later.
So, to get the id, you would do this:
SELECT to_char(substr(maxid,-30)) id, object_id
FROM your_materialized_view;
You could put that in a view to hide the complexity.
Here is a full example:
Create the base table
DROP TABLE t;
create table t (id,object_id,log_cre_date) as
select 1,2,to_date('18/5/2010, 08:00','dd/mm/yyyy, hh:mi') from dual union all
select 2,2,to_date('18/5/2010, 10:00','dd/mm/yyyy, hh mi') from dual union all
select 3,3,to_date('18/5/2010, 11:00','dd/mm/yyyy, hh mi') from dual union all
select 4,3,to_date('18/5/2010, 12:10','dd/mm/yyyy, hh mi') from dual union all
select 5,4,to_date('18/5/2010, 12:20','dd/mm/yyyy, hh mi') from dual union all
select 6,4,to_date('18/5/2010, 11:30','dd/mm/yyyy, hh mi') from dual;
Add some constraints to allow fast-refresh MV
ALTER TABLE t MODIFY id NOT NULL;
ALTER TABLE t ADD CONSTRAINT t_pk PRIMARY KEY ( id );
Create a snapshot log to enable fast refresh
--DROP MATERIALIZED VIEW LOG ON t;
CREATE MATERIALIZED VIEW LOG ON t WITH ROWID, PRIMARY KEY (OBJECT_ID, LOG_CRE_DATE) INCLUDING NEW VALUES;
Create the materialized view (note presence of COUNT(*) in select-list. Important!
--DROP MATERIALIZED VIEW t_mv;
CREATE MATERIALIZED VIEW t_mv
REFRESH FAST ON COMMIT AS
SELECT max(cast(to_char(t.log_cre_date,'YYYYMMDDHH24MISS') || lpad(t.id,30,'0') as varchar2(80))) maxid,
t.object_id,
COUNT(*) cnt
FROM t
GROUP BY t.object_id;
Test it out
select to_number(substr(maxid,-30)) id, object_id
from t_mv;
+----+-----------+
| ID | OBJECT_ID |
+----+-----------+
| 2 | 2 |
| 4 | 3 |
| 5 | 4 |
+----+-----------+
DELETE FROM t WHERE id = 5;
COMMIT;
select to_number(substr(maxid,-30)) id, object_id
from t_mv;
+----+-----------+
| ID | OBJECT_ID |
+----+-----------+
| 4 | 3 |
| 5 | 4 |
| 1 | 2 | -- Now ID #1 is the latest for object_id 2
+----+-----------+

Answer 2

Maybe this query will run faster:
select object_id, id
from (
select object_id, first_value(id) over(partition by object_id order by log_cre_date) as id
from t
)
group by object_id, id;
Hope it helps!

Answer 3

I went through the restriction but I am not sure if following query will work or not.
Try this and let us know if it works.
Select t.id, t.object_id from
T join
(SELECT
min(log_cre_date) mindt,
t.object_id
FROM
t
GROUP BY
t.object_id) t1
On t.object_id = t1.object_id
And t.log_cre_date = t1.mindt;
Cheers!!