Sequence - Oracle 11g - 12c - 18c:

1. Oracle 11g - Sequence and Trigger for auto increment columns :

CREATE TABLE T1 ( ID NUMBER(10) NOT NULL,

DESCRIPTION VARCHAR2(50) NOT NULL);

ALTER TABLE T1 ADD ( CONSTRAINT T1_pk PRIMARY KEY (ID));

CREATE SEQUENCE  SEQ_11g START WITH 1;

CREATE OR REPLACE TRIGGER T1_trg

BEFORE INSERT ON T1

FOR EACH ROW

BEGIN

SELECT SEQ_11g.NEXTVAL

INTO :new.id

FROM dual;

END;

/

2. IDENTITY column on Oracle 12c:

CREATE TABLE  T1 ( c1 NUMBER GENERATED by default on null as IDENTITY,

c2 VARCHAR2(10));

or specify starting and increment values, also preventing any insert into the identity column (GENERATED ALWAYS) (again, Oracle 12c+ only)

CREATE TABLE  T1 ( 

c1 NUMBER GENERATED ALWAYS as IDENTITY(START with 1 INCREMENT by 1),

c2 VARCHAR2(10));

Alternatively, Oracle 12 also allows to use a sequence as a default value:

CREATE SEQUENCE SEQ_12c START WITH 1;

CREATE TABLE T1 (

ID NUMBER(10) DEFAULT SEQ_12c.nextval NOT NULL,

DESCRIPTION VARCHAR2(50) NOT NULL);

ALTER TABLE T1 ADD ( CONSTRAINT T1_pk PRIMARY KEY (ID));

3. Oracle 18c - scalable sequence

Now in Oracle 18c, in those cases of data ingestion workloads with high level of concurrency, the new scalable sequence by generating unordered primary or unique key values helps in significantly reducing the sequence and index block contention caused by the right-handed indexes and thus provides better throughput, data load scalability and performance as compared to the pre -Oracle 19c solution of having to configuring a very large sequence cache using the CACHE clause of CREATE SEQUENCE  or ALTER SEQUENCE statement.

The scalable sequence is made up of a sequence offset number which by default contains 6 digits. The first 3 digits is derived from the instance number with 100 added and the next 3 digits is derived from the SID of that session.

create sequence seq_test_11g

start with 1 increment by 1

maxvalue 100; 

select seq_test_11g.nextval from dual;  

NEXTVAL  :    1 

create sequence seq_test_18c

start with 1 increment by 1

maxvalue 100

scale extend; >>>> (SID nnn, Instance_Number 001)

SQL> select seq_test_18c.nextval from dual;    

NEXTVAL :  101608001

SQL> select instance_number from v$instance; 

INSTANCE_NUMBER :      1 

SQL> select sid from v$mystat where rownum = 1;  

SID :    608

select sequence_name, scale_flag, extend_flag from user_sequences  

where sequence_name='SEQ_TEST_18c';

SEQ_TEST_18c                                                  Y Y

SEQ_TEST_11g                                                  N N