Add position column based on order by - Oracle - sql

Question

I have this table with the following records:
table1
id ele_id_1 ele_val ele_id_2
1 2 123 1
1 1 abc 1
1 4 xyz 2
1 4 456 1
2 5 22 1
2 4 344 1
2 3 6 1
2 2 Test Name 1
2 1 Hello 1
I am trying to add position for each id when ele_id_1 and ele_id_2 is order by ASC.
Here is the output:
id ele_id_1 ele_val ele_id_2 position
1 2 123 1 2
1 1 abc 1 1
1 4 xyz 2 4
1 4 456 1 3
2 5 22 1 5
2 4 344 1 4
2 3 6 1 3
2 2 Test Name 1 2
2 1 Hello 1 1
I have 34 million rows in table1, so would like to use an efficient way of doing this.
Any idea on how I can add position with values?

Answer 1

I think you want row_number() used like this:
select row_number() over (partition by id
order by ele_id_1, ele_id_2
) as position
Oracle can use an index for this, on (id, ele_id_1, ele_id_2).
I should note that for your example data order by ele_id_1, ele_id_2 and order by ele_id_2, ele_id_1 produce the same result. Your question suggests that you want the first.
So, you would get
id ele_id_1 ele_val ele_id_2 position
1 1 123 2 2
1 1 abc 1 1
1 4 xyz 2 4
1 4 456 1 3
Rather than:
id ele_id_1 ele_val ele_id_2 position
1 1 123 2 3
1 1 abc 1 1
1 4 xyz 2 4
1 4 456 1 2
EDIT:
If you want to update the data, then merge is probably the best approach.
MERGE INTO <yourtable> dest
USING (select t.*,
row_number() over (partition by id
order by ele_id_1, ele_id_2
) as new_position
from <yourtable> t
) src
ON dest.id = src.id AND
dest.ele_id_1 = src.ele_id_1 AND
dest.ele_id_2 = src.ele_id_2
WHEN MATCHED THEN UPDATE
SET desc.postition = src.new_position;
Note that updating all the rows in a table is an expensive operation. Truncating the table and recreating it might be easier:
create table temp_t as
select t.*,
row_number() over (partition by id
order by ele_id_1, ele_id_2
) as new_position
from t;
truncate table t;
insert into t ( . . . )
select . . . -- all columns but position
from temp_t;
However, be very careful if you truncate the table. Be sure to back it up first!