rownum / fetch first n rows - sql

Question

select * from Schem.Customer
where cust='20' and cust_id >= '890127'
and rownum between 1 and 2 order by cust, cust_id;
Execution time appr 2 min 10 sec
select * from Schem.Customer where cust='20'
and cust_id >= '890127'
order by cust, cust_id fetch first 2 rows only ;
Execution time appr 00.069 ms
The execution time is a huge difference but results are the same. My team is not adopting to later one. Don't ask why.
So what is the difference between Rownum and fetch first 2 rows and what should I do to improve or convince anyone to adopt.
DBMS : DB2 LUW

Answer 1

Although both SQL end up giving same resultset, it only happens for your data. There is a great chance that resultset would be different. Let me explain why.
I will make your SQL a little simpler to make it simple to understand:
SELECT * FROM customer
WHERE ROWNUM BETWEEN 1 AND 2;
In this SQL, you want only first and second rows. That's fine. DB2 will optimize your query and never look rows beyond 2nd. Because only first 2 rows qualify your query.
Then you add ORDER BY clause:
SELECT * FROM customer
WHERE ROWNUM BETWEEN 1 AND 2;
ORDER BY cust, cust_id;
In this case, DB2 first fetches 2 rows then order them by cust and cust_id. Then sends to client(you). So far so good. But what if you want to order by cust and cust_id first, then ask for first 2 rows? There is a great difference between them.
This is the simplified SQL for this case:
SELECT * FROM customer
ORDER BY cust, cust_id
FETCH FIRST 2 ROWS ONLY;
In this SQL, ALL rows qualify the query, so DB2 fetches all of the rows, then sorts them, then sends first 2 rows to client.
In your case, both queries give same results because first 2 rows are already ordered by cust and cust_id. But it won't work if first 2 rows would have different cust and cust_id values.
A hint about this is FETCH FIRST n ROWS comes after order by, that means DB2 orders the result then retrieves first n rows.

Answer 2

Excellent answer here:
https://blog.dbi-services.com/oracle-rownum-vs-rownumber-and-12c-fetch-first/
Now the index range scan is chosen, with the right cardinality estimation.
So which solution it the best one? I prefer row_number() for several reasons:
I like analytic functions. They have larger possibilities, such as setting the limit as a percentage of total number of rows for example.
11g documentation for rownum says:
The ROW_NUMBER built-in SQL function provides superior support for ordering the results of a query
12c allows the ANSI syntax ORDER BY…FETCH FIRST…ROWS ONLY which is translated to row_number() predicate
12c documentation for rownum adds:
The row_limiting_clause of the SELECT statement provides superior support
rownum has first_rows_n issues as well
PLAN_TABLE_OUTPUT
SQL_ID 49m5a3f33cmd0, child number 0
-------------------------------------
select /*+ FIRST_ROWS(10) */ * from test where contract_id=500
order by start_validity fetch first 10 rows only
Plan hash value: 1912639229
--------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | Buffers |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 | 15 |
|* 1 | VIEW | | 1 | 10 | 10 | 15 |
|* 2 | WINDOW NOSORT STOPKEY | | 1 | 10 | 10 | 15 |
| 3 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 10 | 11 | 15 |
|* 4 | INDEX RANGE SCAN | TEST_PK | 1 | | 11 | 4 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("from$_subquery$_002"."rowlimit_$$_rownumber" <=10)
2 - filter(ROW_NUMBER() OVER ( ORDER BY "TEST"."START_VALIDITY") <=10 )
4 - access("CONTRACT_ID"=500)