PostgreSQL Oracle 兼容性之 - rownum
摘要： 標(biāo)簽 PostgreSQL , rownum , Oracle 兼容性 , row_number 窗口 , limit , PPAS , EDB 背景 Oracle ROWNUM是一個(gè)虛擬列，每輸出一行遞增1。
點(diǎn)此查看原文：https://yq.aliyun.com/articles/405183?spm=a2c4e.11153959.teamhomeleft.24.8WKxt7
標(biāo)簽
PostgreSQL , rownum , Oracle 兼容性 , row_number 窗口 , limit , PPAS , EDB
背景
Oracle ROWNUM是一個(gè)虛擬列，每輸出一行遞增1。
Oracle rownum
通常被用于LIMIT輸出記錄數(shù)。
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(méi)有rownum虛擬列，但是實(shí)現(xiàn)同樣的功能確很容易：
1、輸出行號(hào)，使用臨時(shí)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(hào)，使用窗口函數(shù)
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(yǔ)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(gè)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(fā)行版本PPAS完全實(shí)現(xiàn)了與Oracle一樣的rownum功能。
小結(jié)
主要看業(yè)務(wù)到底用Oracle的rownum干什么，才能因地制宜，使用對(duì)應(yīng)的PG語(yǔ)法來(lái)支持對(duì)應(yīng)的場(chǎng)景。
當(dāng)然，目前PG的商用發(fā)行版本PPAS（EnterpriseDB）的rownum功能已經(jīng)完全和Oracle一樣了。如果業(yè)務(wù)上要節(jié)約改造時(shí)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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