Row returned by a query with ROWNUM and PRIMARY KEY column - sql

Question

I'm reading about 1 of the oracle pseudocolumns i.e
ROWNUM: which return number indicating the order in which oracle return the row.
I have encountered some behavior here,
Example used
1. Create Script:
CREATE TABLE CUSTOMERS(
ID INT NOT NULL,
NAME VARCHAR (20) NOT NULL,
AGE INT NOT NULL,
ADDRESS CHAR (25) ,
SALARY DECIMAL (18, 2),
PRIMARY KEY (ID)
);
2 Insert Script:
INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (2, 'Max', 22, 'India', 4500.00 );
INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (1, 'Maths', 22, 'US', 4500.00 );
select * from CUSTOMERS; executed in the same order as inserted above,
ID NAME AGE ADDRESS SALARY
------ ---- --- ------- ------
2 Max 22 India 4500.00
1 Math 22 US 4500.00
Here if I run the select query,
select
rownum,
customers.ID
from customers;
I get below output:
ROWNUW ID
------ --
1 1
2 2
Here ID = 2 is inserted first but oracle returns in the 2nd row,
But if you include any other column from a table with ROWNUM and PK like
select
rownum,
customers.ID,customers.Name
from customers;
I get correct output (Correct inserted order) :
ROWNUW ID NAME
------ -- ----
1 2 Max
2 1 Math
If run query without Name column i.e only ROWNUW (pseudocolumns) and PK (table column)
We get this,
ROWNUW ID
------ --
1 1
2 2
My Question is Why is that ID=2 is not first returned row.
if I query any table column with ROWNUM, I get the result back based on insertion order.
Example below
select
rownum,
(NAME / AGE / ADDRESS SALARY) any one of these columns
From CUSTOMERS
But if use ROWNUM with ID (Primary Key column) insertion order is not working. why is this behavior with the only Primary Key column?

Answer 1

ROWNUM values depends on how oracle access the resultset. Once resultset is fetched, rownum is assigned to the rows.
As there is no guarantee in what order the data is returned, there is no guarantee in what order rownum is assigned.
Maybe if you try running both of your queries without rownum, you might get the rows swapped as you showed, or they might not.

Answer 2

Rownum will always be in order of 1,2,3,4 .. no matter what you are getting in result set. It is always calculated after result set is returned from query.
Now, you have ID column as PRIMARY KEY which makes it eligible for default clustered index creation in database. So when you select only PRIMARY column, it will be sorted as it was stored in DB that way. Index created as a result of PRIMARY key creation is sorted in ASC which is default.
Here if you select only PRIMARY KEY column i.e. "ID", Database is returning it in sorted order and ROWNUM is writing 1 & 2 to it.
But if you are selecting some extra non-PRIMARY KEY as well then it is totally upto oracle, whichever way it finds data faster. Sometimes it will return data from cache memory. If not it will read disk and fetch data. While reading datafile, there are multiple things which will differ machine to machine i.e is your table partitioned? is your table sharing tablespace ? what organization you are using in table ? blah blah....
Bottom line is, don't trust ROWNUM. It gives 1,2,3.. after you have applied your brain to the query.