why adding order by in the query changes the aggregate value? - vertica

Question

Following vertica example from https://www.vertica.com/docs/11.0.x/HTML/Content/Authoring/AnalyzingData/SQLAnalytics/AnalyticFunctionsVersusAggregateFunctions.htm?tocpath=Analyzing%20Data%7CSQL%20Analytics%7C_____2
CREATE TABLE employees(emp_no INT, dept_no INT);
INSERT INTO employees VALUES(1, 10);
INSERT INTO employees VALUES(2, 30);
INSERT INTO employees VALUES(3, 30);
INSERT INTO employees VALUES(4, 10);
INSERT INTO employees VALUES(5, 30);
INSERT INTO employees VALUES(6, 20);
INSERT INTO employees VALUES(7, 20);
INSERT INTO employees VALUES(8, 20);
INSERT INTO employees VALUES(9, 20);
INSERT INTO employees VALUES(10, 20);
INSERT INTO employees VALUES(11, 20);
COMMIT;
If I run this query without order by, I get same count value for all rows
dbadmin#b006bc38a718(*)=>
select
emp_no
, dept_not
, count(*) over (partition by dept_not) as emp_count
from employees;
emp_no | dept_not | emp_count
--------+----------+-----------
6 | 20 | 6
7 | 20 | 6
8 | 20 | 6
9 | 20 | 6
10 | 20 | 6
11 | 20 | 6
1 | 10 | 2
4 | 10 | 2
2 | 30 | 3
3 | 30 | 3
5 | 30 | 3
(11 rows)
But if I add order by, I get incremental value
dbadmin#b006bc38a718(*)=>
select
emp_no
, dept_not
, count(*) over (partition by dept_not order by emp_no) as emp_count
from employees;
emp_no | dept_not | emp_count
--------+----------+-----------
2 | 30 | 1
3 | 30 | 2
5 | 30 | 3
1 | 10 | 1
4 | 10 | 2
6 | 20 | 1
7 | 20 | 2
8 | 20 | 3
9 | 20 | 4
10 | 20 | 5
11 | 20 | 6
(11 rows)
Time: First fetch (11 rows): 85.075 ms. All rows formatted: 85.139 ms
What is the affect of order by ? Why do I get incremental value?

Answer 1

If the window clause only contains PARTITION BY, it returns the total sum of the partition - for each row of the partition the same value.
If the window clause contains both PARTITION BY and ORDER BY, it returns the running count within the partition . So, using the ORDER BY expression, how many rows have been counted so far within the partition.
That's exactly how window functions work. They give you a whole world of possibilities ...

Answer 2

That happens because Vertica applies a default frame-clause which is defined as:
RANGE UNBOUNDED PRECEDING AND CURRENT ROW
So to get the result you want, you may want to add the frame clause as below after you ORDER BY in the OVER() clause:
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
This behaviour is documented as:
If the OVER clause omits specifying a window frame, the function creates a default window that extends from the current row to the first row in the current partition.
Link to doc