創建序列、同義詞、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 下一個值， CURRVAL 當前值
第一次先執行NEXTVAL，才能對CURRVAL取值。
select detpno.currval from dual;
取值只跟當前會話有關。
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（序列不連續有以下3種情況）:
1、A rollback occurs
2、The system crashes 如：中斷oracle , ps -ef |grep smon -> kill -9 進程號 。 shutdown immediate 關閉數據庫
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
循環到最大值時，起始值再從1開始
要想改起始值，只能刪除序列重建
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;
監控last_number和最大值的差距，差距越小越要注意 ，特別是有序列作爲外鍵時,很重要。
synonyms 同義詞
創建同義詞
CREATE [PUBLIC] SYNONYM synonym
FOR object;
DESCRIBE user_synonyms;
SELECT *
FROM user_synonyms;
示例：
create synonym test for v$SESSION;
只有別名不會分配存儲單元，默認爲私有。
加public 則是全局公有。
示例：
create public synonym test2 for emp;
create departments for hr.department ;
truncate table emp; 刪除表的行內容，表結構還存在。
drop synony emp; 刪除同義詞。
創建索引 create index
1、是用戶對象
2、提高數據庫性能
3、減少I/O
4、依賴於表
5、增刪改表的時候，系統自動更新索引
自動會創建索引，在主鍵和唯一鍵時
Automatically: A unique index is created automatically when you define a PRIMARY KEY or UNIQUE constraint in a table definition.
手動建索引
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 ; 索引不可見；
alter index emp_last_name_idx visible; 索引可見；
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';