Oracle pagination query and user defined function - slow execution - oracle

Question

My query is like this:
select id, fn_calc(col)
from table_a
order by id
offset 1483800 rows fetch next 100 rows only;
Note that the paging offset is extremely high - table_a has about 1,500,000 rows.
The above query takes very long, but when fn_calc(col) is replaced by col, query speed is satisfactory - at least 5 times faster. But when the offset is 0 or 100, two queries are almost equally fast. Why this difference?
Possible reasons I can think of:
Oracle executes fn_calc() 1483900 times, although it is logically not neccessary. (It is enough to call it only 100 times)
The calling cost for a user function in a query is very high.
I'm using Oracle 12c on ExaData.
Any suggestion can help.
UPDATE:
When the above query is changed as follows:
select id, fn_calc(col)
from
(
select id, col
from table_a
order by id
offset 1483800 rows fetch next 100 rows only
)
order by id
The query speed is comparable to the case when fn_calc() is not called at all.
Why this?
UPDATE:
The execution plans are as follows: (Sorry, currently only I have is SQLDevelper, so I had to capture the result.)
The first query:
The second query (which uses subquery):
To down-voters and close-voters: Please specify your concerns about this question before you vote. I'll update my question accordingly. This question is about the real and serious problem to me. Please do not deprive the chance to get help.

Answer 1

Firstly, what is the purpose of the pagination query when you are not using ORDER BY clause. You are randomly fetching the rows, i.e. Oracle will internally apply ORDER BY NULL.
Secondly, since you have the function in the SELECT and not in the filter predicate, the explain plan should be same. The only extra time spent should be due to the function.
For example,
SQL> CREATE OR REPLACE FUNCTION f_char(
2 i_empno NUMBER)
3 RETURN VARCHAR2
4 AS
5 v_empno VARCHAR2(10);
6 BEGIN
7 v_empno := TO_CHAR(i_empno);
8 return v_empno;
9 END;
10 /
Function created.
Let's compare the explain plan:
SQL> set autot on explain
SQL>
SQL> SELECT f_char(empno) FROM emp
2 OFFSET 5 ROWS
3 FETCH NEXT 5 rows only;
F_CHAR(EMPNO)
--------------------------------------------------------------------------------
7698
7782
7788
7839
7844
Execution Plan
----------------------------------------------------------
Plan hash value: 3611411408
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 28210 | 4 (0)| 00:00:01 |
|* 1 | VIEW | | 14 | 28210 | 4 (0)| 00:00:01 |
|* 2 | WINDOW NOSORT STOPKEY| | 14 | 56 | 4 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | EMP | 14 | 56 | 4 (0)| 00:00:01 |
-------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("from$_subquery$_002"."rowlimit_$$_rownumber"<=CASE WHEN
(5>=0) THEN 5 ELSE 0 END +5 AND "from$_subquery$_002"."rowlimit_$$
_rownu
mber">5)
2 - filter(ROW_NUMBER() OVER ( ORDER BY NULL )<=CASE WHEN (5>=0)
THEN 5 ELSE 0 END +5)
SQL> SELECT empno FROM emp
2 OFFSET 5 ROWS
3 FETCH NEXT 5 rows only;
EMPNO
----------
7698
7782
7788
7839
7844
Execution Plan
----------------------------------------------------------
Plan hash value: 3611411408
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 364 | 4 (0)| 00:00:01 |
|* 1 | VIEW | | 14 | 364 | 4 (0)| 00:00:01 |
|* 2 | WINDOW NOSORT STOPKEY| | 14 | 56 | 4 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | EMP | 14 | 56 | 4 (0)| 00:00:01 |
-------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("from$_subquery$_002"."rowlimit_$$_rownumber"<=CASE WHEN
(5>=0) THEN 5 ELSE 0 END +5 AND "from$_subquery$_002"."rowlimit_$$
_rownu
mber">5)
2 - filter(ROW_NUMBER() OVER ( ORDER BY NULL )<=CASE WHEN (5>=0)
THEN 5 ELSE 0 END +5)