Oracle_071_lesson_p13

Posted 2020-12-28

创建序列、同义词、索引

create sequence
CREATE SEQUENCE [ schema. ] sequence
[ { START WITH|INCREMENT BY } integer
| { MAXVALUE integer | NOMAXVALUE }
| { MINVALUE integer | NOMINVALUE }
| { CYCLE | NOCYCLE }
| { CACHE integer | NOCACHE }
| { ORDER | NOORDER }
];

CREATE SEQUENCE dept_deptid_seq
START WITH 280
INCREMENT BY 10
MAXVALUE 9999
NOCACHE
NOCYCLE;

INSERT INTO departments(department_id,
department_name, location_id)
VALUES (dept_deptid_seq.NEXTVAL,
‘Support‘, 2500);

SELECT dept_deptid_seq.CURRVAL
FROM dual;

NEXTVAL 下一个值， CURRVAL 当前值
第一次先执行NEXTVAL，才能对CURRVAL取值。
select detpno.currval from dual;
取值只跟当前会话有关。

示例：
CREATE SEQUENCE s1 START WITH 1;
CREATE TABLE emp (a1 NUMBER DEFAULT s1.NEXTVAL NOT NULL, a2 VARCHAR2(10));
INSERT INTO emp (a2) VALUES (‘john‘);
INSERT INTO emp (a2) VALUES (‘mark‘);
SELECT * FROM emp;

Caching sequence values in memory gives faster access to those values.
Gaps in sequence values can occur when（序列不连续有以下3种情况）:
1、A rollback occurs
2、The system crashes 如：中断oracle , ps -ef |grep smon -> kill -9 进程号 。 shutdown immediate 关闭数据库
3、A sequence is used in another table

修改序列 alter sequence
ALTER SEQUENCE dept_deptid_seq
INCREMENT BY 20
MAXVALUE 999999
NOCACHE
NOCYCLE;

DROP SEQUENCE dept_deptid_seq;

start with 起始值不能改
循环到最大值时，起始值再从1开始
要想改起始值，只能删除序列重建

DESCRIBE user_sequences
SELECT sequence_name, min_value, max_value,
increment_by, last_number
FROM user_sequences;

查看序列信息： select * from user_sequences;
监控last_number和最大值的差距，差距越小越要注意 ，特别是有序列作为外键时,很重要。

synonyms 同义词

创建同义词
CREATE [PUBLIC] SYNONYM synonym
FOR object;

DESCRIBE user_synonyms;

SELECT *
FROM user_synonyms;

示例：
create synonym test for v$SESSION;
只有别名不会分配存储单元，默认为私有。
加public 则是全局公有。

示例：
create public synonym test2 for emp;
create departments for hr.department ;
truncate table emp; 删除表的行内容，表结构还存在。
drop synony emp; 删除同义词。

创建索引 create index

1、是用户对象
2、提高数据库性能
3、减少I/O
4、依赖于表
5、增删改表的时候，系统自动更新索引

自动会创建索引，在主键和唯一键时
Automatically: A unique index is created automatically when you define a PRIMARY KEY or UNIQUE constraint in a table definition.

手动建索引
Manually: You can create unique or nonunique index on columns to speed up access to the rows.

create index emp_last_name_idx
on table (column1,column2.......);

DROP INDEX index;
drop index emp_last_name_idx;
alter index emp_last_name_idx invisible ; 索引不可见；
alter index emp_last_name_idx visible; 索引可见；

CREATE TABLE NEW_EMP
(employee_id NUMBER(6) PRIMARY KEY USING INDEX
(CREATE INDEX emp_id_idx ON NEW_EMP(employee_id)),
first_name VARCHAR2(20),
last_name VARCHAR2(25));

SELECT INDEX_NAME, TABLE_NAME
FROM USER_INDEXES
WHERE TABLE_NAME = ‘NEW_EMP‘;

CREATE INDEX emp_id_name_ix1
ON employees(employee_id, first_name);

ALTER INDEX emp_id_name_ix1 INVISIBLE;

CREATE BITMAP INDEX emp_id_name_ix2
ON employees(employee_id, first_name);

DESCRIBE user_indexes;

DESCRIBE user_ind_columns;

SELECT index_name, column_name,table_name FROM user_ind_columns WHERE index_name = ‘LNAME_IDX‘;