how to query by row number in oracle - oracle11g

Question

Is there any way to query directly by row number in a table in Oracle? In other words, to achieve the same effect of ordinary lookup in an array in some basic language like C or Java. I've not yet tried virtual columns.
For instance, the following is an example of an efficient query, but it wastes disk space:
create table ary (row_position_id number(10) NOT NULL,
datum binary_float NOT NULL);
declare i pls_integer;
begin
for i in 0..10000000
loop
insert into ary values (i, dbms_random.normal());
end loop;
commit;
end;
create unique index ary_rp on ary(row_position_id);
now, i'm going to create a set of query values to store in another "parameter" table:
create table query_values (qval number(10) NOT NULL);
declare i pls_integer;
begin
for i in 0..10000
loop
insert into query_values (abs(dbms_random.random() % 10000000));
end loop;
commit;
end;
now, having these query values, i'm going to query the original table
select d.* from ary d where exists (select 0 from query_values v
where d.row_position_id = v.qval);
Now, this query would be fine -- it would use INDEX UNIQUE SCAN and TABLE access by ROWID. The problem I have is that the row_position_id takes up as much space in the table blocks as the actual data (the DATUM column).
I am aware of Index-organized tables and also Virtual Columns (which cannot be used with IOTs). And, of course, things like ROWNUM and ROW_NUMBER are irrelevant here (unless I'm misunderstanding something).
Also worth pointing out, this table is static data -- once loaded, it will never change. I would likely do an ALTER TABLE ARY READ ONLY;
What I would really like is:
create table ary (datum binary_float not null);
-- load rows in a specific order
-- efficiently query this table by implicit row position
Thanks very much!
Henry

Answer 1

I think you're going to want to keep the extra column. Here's why:
As you said, ROWNUM and ROW_NUMBER are not applicable here because they are generated as rows are returned in the query; they will not tell you anything about insert order.
What about ROWID? ROWID is just where a row is stored - again, from the docs:
The data object number of the object
The data block in the data file in which the row resides
The position of the row in the data block (first row is 0)
The data file in which the row resides (first file is 1). The file number is relative to the tablespace.
The "position in the data block" sounds interesting, but you would have no idea what order of the data blocks that were inserted (Oracle could use whatever datablocks it can quickly make use of) so this would not be a reliable option, and even so, you'd be having to parse ROWIDs which are not human readable (e.g. in 12g they look like this: *BAGAASMCwQL+ )
Another option is ORA_ROWSCN which is interesting in that it does give you some idea of order, in terms of the system change number. However, it doesn't come for free. Just to start, you have to create your table with the ROWDEPENDENCIES option and as per docs:
ROWDEPENDENCIES Specify ROWDEPENDENCIES if you want to enable
row-level dependency tracking. This setting is useful primarily to
allow for parallel propagation in replication environments. It
increases the size of each row by 6 bytes.
The other catch with this is that you would have to have to follow each row inserted with a commit so each row would get a different SCN.
If you're willing to go this far, you'll still have to convert the rows to have indexes (starting with, say, 0 or 1) that you can use to join to other tables.
Here's a quick sample of what it would involve:
DROP TABLE temp;
CREATE TABLE temp
( a number(10)
, b varchar2(10)
)
ROWDEPENDENCIES
;
-- one commit after all rows
INSERT INTO temp VALUES (1, 'A');
INSERT INTO temp VALUES (2, 'B');
INSERT INTO temp VALUES (3, 'C');
INSERT INTO temp VALUES (4, 'D');
INSERT INTO temp VALUES (5, 'E');
INSERT INTO temp VALUES (6, 'F');
COMMIT;
SELECT X.*, ROWNUM
FROM (SELECT T.*
, ORA_ROWSCN
FROM TEMP T
ORDER BY ORA_ROWSCN
) x
;
A B ORA_ROWSCN ROWNUM
1 A 2272340 1
2 B 2272340 2
6 F 2272340 3
4 D 2272340 4
5 E 2272340 5
3 C 2272340 6
Whoops. Those rows are definitely not in the order they came in.
Now using one commit per row:
TRUNCATE TABLE temp;
INSERT INTO temp VALUES (1, 'A');
COMMIT;
INSERT INTO TEMP VALUES (2, 'B');
COMMIT;
INSERT INTO temp VALUES (3, 'C');
COMMIT;
INSERT INTO temp VALUES (4, 'D');
COMMIT;
INSERT INTO temp VALUES (5, 'E');
COMMIT;
INSERT INTO temp VALUES (6, 'F');
COMMIT;
SELECT X.*, ROWNUM
FROM (SELECT T.*
, ORA_ROWSCN
FROM TEMP T
ORDER BY ORA_ROWSCN
) x
;
A B ORA_ROWSCN ROWNUM
1 A 2272697 1
2 B 2272699 2
3 C 2272701 3
4 D 2272703 4
5 E 2272705 5
6 F 2272707 6
Better. But if you've got a significant number of rows it's not going to go in fast. (I think this is what you would do if you intentionally wanted to slow down your inserts. ;) )
I think that's about as good as you'll get trying to get around using your own column, BUT there is still hope to economize storage: you can do away with the table + index and just go with an index-organized table. It's basically an index that you query directly.
It's just this easy:
CREATE TABLE TEMP2
( A NUMBER(10)
, B VARCHAR2(10)
, CONSTRAINT PK_CONSTRAINT PRIMARY KEY (A)
)
ORGANIZATION INDEX
;
There are other parameters you'll want to consider for this as well, but for more info check out... the docs.