The document discusses pagination techniques in SQL, primarily focusing on how to retrieve ordered subsets of data from a database. It includes various SQL query examples for selecting employee records ordered by hire date, and illustrates methods to improve query efficiency such as using window functions and caching. The author encourages sharing knowledge and offers resources for further learning on database topics.

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Ask TOM Mini-Lesson
Pagination with Connor McDonald
Confidential – Oracle Internal/Restricted/Highly Restricted

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Connor McDonald

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150 hours of free access (so far)
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10
PAGINATION

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not really...
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12
"source from the database an ordered
subset of an entire dataset"

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seems so simple ...
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"employees by hiredate, recent first"

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SQL> select empno, ename, hiredate
2 from emp
3 where rownum <= 5
4 order by hiredate desc;
EMPNO ENAME HIREDATE
---------- ---------- -------------------
7654 MARTIN 28/09/1981 00:00:00
7566 JONES 02/04/1981 00:00:00
7521 WARD 22/02/1981 00:00:00
7499 ALLEN 20/02/1981 00:00:00
7369 SMITH 17/12/1980 00:00:00
15

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inline view
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SQL> select *
2 from (
3 select empno, ename, hiredate
4 from emp
5 order by hiredate desc
6 )
7 where rownum <= 5;
EMPNO ENAME HIREDATE
---------- ---------- ---------
7876 ADAMS 12-JAN-83
7788 SCOTT 09-DEC-82
7934 MILLER 23-JAN-82
7900 JAMES 03-DEC-81
7902 FORD 03-DEC-81
17

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SQL> select *
2 from (
3 select
4 empno, ename, hiredate,
5 row_number() over ( order by hiredate desc) rn
6 from emp
7 )
8 where rn <= 5;
18

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SQL> select empno, ename, hiredate
2 from emp
3 order by hiredate desc
4 fetch first 5 rows only;
EMPNO ENAME HIREDATE
---------- ---------- ---------
7876 ADAMS 12-JAN-83
7788 SCOTT 09-DEC-82
7934 MILLER 23-JAN-82
7900 JAMES 03-DEC-81
7902 FORD 03-DEC-81
19

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20
"TL;DR ... my app can do it"

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public static void Paging(Connection conn ) throws Exception
{
PreparedStatement sql_stmt =
conn.prepareStatement(
"select empno, ename, hiredate
from emp
order by hiredate desc");
ResultSet rset = sql_stmt.executeQuery();
int i = 0;
while( rset.next() )
{
...
i = i + 1;
if (i > 5) {
break;
}
}
rset.close();
}
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demo
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oc01.sql

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let the database know
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SQL> select *
2 from (
3 select empno, ename, hiredate
4 from emp
5 order by hiredate desc
6 )
7 where rownum <= 5;
25
------------------------------------------------
| Id | Operation | Name | Rows |
------------------------------------------------
| 0 | SELECT STATEMENT | | 5 |
|* 1 | COUNT STOPKEY | | |
| 2 | VIEW | | 14 |
|* 3 | SORT ORDER BY STOPKEY| | 14 |
| 4 | TABLE ACCESS FULL | EMP | 14 |
------------------------------------------------

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SQL> select empno, ename, hiredate
2 from emp
3 order by hiredate desc
4 fetch first 5 rows only;
26
-------------------------------------------------
| Id | Operation | Name | Rows |
-------------------------------------------------
| 0 | SELECT STATEMENT | | 14 |
|* 1 | VIEW | | 14 |
|* 2 | WINDOW SORT PUSHED RANK| | 14 |
| 3 | TABLE ACCESS FULL | EMP | 14 |
-------------------------------------------------

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you get other benefits
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oc02.sql

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28
"but what about the next page ?"

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there
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is
no
next
page

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new query
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SQL> select empno, ename, hiredate
2 from emp
3 order by hiredate desc
4 offset 5 rows fetch first 5 rows only;
EMPNO ENAME HIREDATE
---------- ---------- ---------
7839 KING 17-NOV-81
7654 MARTIN 28-SEP-81
7844 TURNER 08-SEP-81
7782 CLARK 09-JUN-81
7698 BLAKE 01-MAY-81
31

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forget the OFFSET clause
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... maybe
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SQL> select empno, ename, hiredate
2 from emp
3 order by hiredate desc;
HIREDATE EMPNO ENAME
--------- ---------- --------
12-JAN-83 7876 ADAMS
09-DEC-82 7788 SCOTT
23-JAN-82 7934 MILLER
03-DEC-81 7902 FORD
03-DEC-81 7900 JAMES
17-NOV-81 7839 KING
28-SEP-81 7654 MARTIN
08-SEP-81 7844 TURNER
09-JUN-81 7782 CLARK
01-MAY-81 7698 BLAKE
02-APR-81 7566 JONES
22-FEB-81 7521 WARD
20-FEB-81 7499 ALLEN
17-DEC-80 7369 SMITH
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fetch first 5 rows
offset 5 fetch next ...
17-FEB-91 7521 BROWN

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SQL> select empno, ename, hiredate
2 from emp
3 order by hiredate desc;
HIREDATE EMPNO ENAME
--------- ---------- --------
12-JAN-83 7876 ADAMS
09-DEC-82 7788 SCOTT
23-JAN-82 7934 MILLER
03-DEC-81 7902 FORD
03-DEC-81 7900 JAMES
17-NOV-81 7839 KING
28-SEP-81 7654 MARTIN
08-SEP-81 7844 TURNER
09-JUN-81 7782 CLARK
01-MAY-81 7698 BLAKE
02-APR-81 7566 JONES
22-FEB-81 7521 WARD
20-FEB-81 7499 ALLEN
17-DEC-80 7369 SMITH
35
17-FEB-91 7521 BROWN
SQL> select empno, ename, hiredate
2 from emp
3 where hiredate < :last_shown
3 order by hiredate desc;

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36
"an expensive query per page ?!?!

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consider result caching
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SQL> with first_200 as
2 ( select f.*, rownum r
3 from
4 ( select *
5 from t
6 order by owner, object_name desc
7 ) f
8 where rownum <= 200
9 )
10 select *
11 from first_200
12 where r <= 10
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/*+ result_cache */ rownum r, f.*
oc03.sql
r between 11 and 20

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wrap up
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share your knowledge with the db

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SQL generally beats the App
41

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