摘要： 标签 PostgreSQL , rownum , Oracle 兼容性 , row_number 窗口 , limit , PPAS , EDB 背景 Oracle ROWNUM是一个虚拟列，每输出一行递增1。
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标签
PostgreSQL , rownum , Oracle 兼容性 , row_number 窗口 , limit , PPAS , EDB
背景
Oracle ROWNUM是一个虚拟列，每输出一行递增1。
Oracle rownum
通常被用于LIMIT输出记录数。
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 目前没有rownum虚拟列，但是实现同样的功能确很容易：
1、输出行号，使用临时序列
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、输出行号，使用窗口函数
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，直接语法支持
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、为某个字段生成序列值。
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的商用发行版本PPAS完全实现了与Oracle一样的rownum功能。
小结
主要看业务到底用Oracle的rownum干什么，才能因地制宜，使用对应的PG语法来支持对应的场景。
当然，目前PG的商用发行版本PPAS（EnterpriseDB）的rownum功能已经完全和Oracle一样了。如果业务上要节约改造时间，使用PPAS是最好的选择。
参考
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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