Oracle sequence的基本概念与理解

Posted 2020-10-30 dayu.liu

1.如何查看sequence的定义

2.dba_sequences相关字的定义

3.如何修改sequence

**********************************************************************************************************************************************************

**********************************************************************************************************************************************************

1.如何查看sequence的定义

SQL>SELECT DBMS_METADATA.GET_DDL(‘SEQUENCE‘,‘SEQ_SMECUSTPERIODINFO‘,‘CLMS01‘) FROM DUAL;

CREATE SEQUENCE "CLMS01"."SEQ_SMECUSTPERIODINFO" MINVALUE 1 MAXVALUE 999999
9999 INCREMENT BY 1 START WITH 8798675575 CACHE 20 ORDER NOCYCLE  --START WITH 8798675575 此值为dba_sequneces中的LAST_NUMBER，比较奇怪！！

2.dba_sequences相关字的定义

Column	Datatype	NULL	Description
SEQUENCE_OWNER	VARCHAR2(30)	NOT NULL	Owner of the sequence
SEQUENCE_NAME	VARCHAR2(30)	NOT NULL	Sequence name
MIN_VALUE	NUMBER		Minimum value of the sequence
MAX_VALUE	NUMBER		Maximum value of the sequence
INCREMENT_BY	NUMBER	NOT NULL	Value by which sequence is incremented
CYCLE_FLAG	VARCHAR2(1)		Indicates whether the sequence wraps around on reaching the limit (Y) or not (N)
ORDER_FLAG	VARCHAR2(1)		Indicates whether sequence numbers are generated in order (Y) or not (N)
CACHE_SIZE	NUMBER	NOT NULL	Number of sequence numbers to cache
LAST_NUMBER	NUMBER	NOT NULL	Last sequence number written to disk. If a sequence uses caching, the number written to disk is the last number placed in the sequence cache. This number is likely to be greater than the last sequence number that was used.

• CYCLE_FLAG：标志sequence是否循环使用（到最大值之后变为最小值）
• ORDER_FLAG：标志sequence是否是按照顺序生成。
• CACHE_SIZE：标志内存中缓存的sequence个数（保存在shared pool中），设置较小时会出现row cache lock的等待事件。
• LAST_NUMBER：不太好理解。

3.如何修改sequence