摘要： 標簽 PostgreSQL , rownum , Oracle 兼容性 , row_number 窗口 , limit , PPAS , EDB 背景 Oracle ROWNUM是一個虛擬列，每輸出一行遞增1。

點此查看原文：https://yq.aliyun.com/articles/405183?spm=a2c4e.11153959.teamhomeleft.24.8WKxt7

標簽

PostgreSQL , rownum , Oracle 兼容性 , row_number 窗口 , limit , PPAS , EDB

背景

Oracle ROWNUM是一個虛擬列，每輸出一行遞增1。

Oracle rownum

通常被用于LIMIT輸出記錄數。

SELECT ROWNUM, empno, ename, job FROM emp WHERE ROWNUM < 5 ORDER BY ename;

rownum | empno | ename | job?
——–+——-+——-+———-?
2 | 7499 | ALLEN | SALESMAN?
4 | 7566 | JONES | MANAGER?
1 | 7369 | SMITH | CLERK?
3 | 7521 | WARD | SALESMAN?
(4 rows)?
或者用于生成序列值。

ALTER TABLE jobhist ADD seqno NUMBER(3);?
UPDATE jobhist SET seqno = ROWNUM;?
SELECT seqno, empno, TO_CHAR(startdate,’DD-MON-YY’) AS start, job FROM jobhist;

seqno | empno | start | job?
——-+——-+———–+———–?
1 | 7369 | 17-DEC-80 | CLERK?
2 | 7499 | 20-FEB-81 | SALESMAN?
3 | 7521 | 22-FEB-81 | SALESMAN?
4 | 7566 | 02-APR-81 | MANAGER?
5 | 7654 | 28-SEP-81 | SALESMAN?
6 | 7698 | 01-MAY-81 | MANAGER?
7 | 7782 | 09-JUN-81 | MANAGER?
8 | 7788 | 19-APR-87 | CLERK?
9 | 7788 | 13-APR-88 | CLERK?
10 | 7788 | 05-MAY-90 | ANALYST?
11 | 7839 | 17-NOV-81 | PRESIDENT?
12 | 7844 | 08-SEP-81 | SALESMAN?
13 | 7876 | 23-MAY-87 | CLERK?
14 | 7900 | 03-DEC-81 | CLERK?
15 | 7900 | 15-JAN-83 | CLERK?
16 | 7902 | 03-DEC-81 | ANALYST?
17 | 7934 | 23-JAN-82 | CLERK?
(17 rows)

PostgreSQL rownum

PostgreSQL 目前沒有rownum虛擬列，但是實現同樣的功能確很容易：

1、輸出行號，使用臨時序列

postgres=# create temp sequence if not exists tmp_seq;

postgres=# alter sequence tmp_seq restart with 1;

postgres=# select nextval(‘tmp_seq’) as rownum, * from test limit 10;?
rownum | id | info | crt_time?
——–+—-+——+—————————-?
1 | 1 | test | 2018-01-24 11:06:24.882708?
2 | 2 | test | 2018-01-24 11:06:24.882708?
3 | 3 | test | 2018-01-24 11:06:24.882708?
4 | 4 | test | 2018-01-24 11:06:24.882708?
5 | 5 | test | 2018-01-24 11:06:24.882708?
6 | 6 | test | 2018-01-24 11:06:24.882708?
7 | 7 | test | 2018-01-24 11:06:24.882708?
8 | 8 | test | 2018-01-24 11:06:24.882708?
9 | 9 | test | 2018-01-24 11:06:24.882708?
10 | 10 | test | 2018-01-24 11:06:24.882708?
(10 rows)?
2、輸出行號，使用窗口函數

postgres=# select row_number() over () as rownum, * from test limit 10;?
rownum | id | info | crt_time?
——–+—-+——+—————————-?
1 | 1 | test | 2018-01-24 11:06:24.882708?
2 | 2 | test | 2018-01-24 11:06:24.882708?
3 | 3 | test | 2018-01-24 11:06:24.882708?
4 | 4 | test | 2018-01-24 11:06:24.882708?
5 | 5 | test | 2018-01-24 11:06:24.882708?
6 | 6 | test | 2018-01-24 11:06:24.882708?
7 | 7 | test | 2018-01-24 11:06:24.882708?
8 | 8 | test | 2018-01-24 11:06:24.882708?
9 | 9 | test | 2018-01-24 11:06:24.882708?
10 | 10 | test | 2018-01-24 11:06:24.882708?
(10 rows)?
3、LIMIT，直接語法支持

postgres=# select * from test limit 10;?
id | info | crt_time?
—-+——+—————————-?
1 | test | 2018-01-24 11:06:24.882708?
2 | test | 2018-01-24 11:06:24.882708?
3 | test | 2018-01-24 11:06:24.882708?
4 | test | 2018-01-24 11:06:24.882708?
5 | test | 2018-01-24 11:06:24.882708?
6 | test | 2018-01-24 11:06:24.882708?
7 | test | 2018-01-24 11:06:24.882708?
8 | test | 2018-01-24 11:06:24.882708?
9 | test | 2018-01-24 11:06:24.882708?
10 | test | 2018-01-24 11:06:24.882708?
(10 rows)?
4、為某個字段生成序列值。

postgres=# create temp sequence if not exists tmp_seq;

postgres=# alter sequence tmp_seq restart with 1;

postgres=# alter table test add column col1 int;?
ALTER TABLE

postgres=# update test set col1=nextval(‘tmp_seq’);?
UPDATE 10000000

postgres=# select * from test limit 10;?
id | info | crt_time | col1?
—-+——+—————————-+——?
1 | test | 2018-01-24 11:06:24.882708 | 1?
2 | test | 2018-01-24 11:06:24.882708 | 2?
3 | test | 2018-01-24 11:06:24.882708 | 3?
4 | test | 2018-01-24 11:06:24.882708 | 4?
5 | test | 2018-01-24 11:06:24.882708 | 5?
6 | test | 2018-01-24 11:06:24.882708 | 6?
7 | test | 2018-01-24 11:06:24.882708 | 7?
8 | test | 2018-01-24 11:06:24.882708 | 8?
9 | test | 2018-01-24 11:06:24.882708 | 9?
10 | test | 2018-01-24 11:06:24.882708 | 10?
(10 rows)

PPAS rownum

https://www.enterprisedb.com/docs/en/10.0/Ora_Compat_Dev_Guide/Database_Compatibility_for_Oracle_Developers_Guide.1.021.html#pID0E0AEK0HA

PG的商用發行版本PPAS完全實現了與Oracle一樣的rownum功能。

小結

主要看業務到底用Oracle的rownum干什么，才能因地制宜，使用對應的PG語法來支持對應的場景。

當然，目前PG的商用發行版本PPAS（EnterpriseDB）的rownum功能已經完全和Oracle一樣了。如果業務上要節約改造時間，使用PPAS是最好的選擇。

參考

https://www.enterprisedb.com/docs/en/10.0/Ora_Compat_Dev_Guide/Database_Compatibility_for_Oracle_Developers_Guide.1.021.html#pID0E0AEK0HA

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