What is Sequence in oracle

Table of Contents

What is Sequence in Oracle?

Oracle Sequence is a user-created object which can be shared by multiple users to generate unique integers

The most general use of sequences would be to generate a primary key column in the table.
The sequence is generated by Oracle’s internal routine so we don’t need to worry about it. It will save time as developers don’t need to generate the sequence-producing routine

A sequential number generator can be configured to increase or decrease
It exists only in the data dictionary and can be limited or repeatable (cycle).
We need to have create sequence privilege to create the sequence

Creating a sequence is done using the

CREATE SEQUENCE   [START WITH] [INCREMENT BY] [NO/MINVALUE] [NO/MAXVALUE] [NO/CYCLE] [NO/CACHE];

Description of each value

Examples

create sequence test_tech start with 1
increment by 1
maxvalue 10000
cycle
cache 20;
Or
create sequence test_tech1
start with 1
increment by 1
maxvalue 4500000
cycle
nocache;

How to use sequences

In order to use sequence simply use the CURRVAL and NEXTVAL pseudo column

NEXTVAL pseudo column

It is used to generate the successive sequence number of the specified sequence

CURRVAL pseudo column

It holds the sequence which the user just generated

SELECT TEST_TECH.NEXTVAL FROM DUAL;
SELECT TEST_TECH.CURRVAL FROM DUAL;
SELECT TEST_TECH.NEXTVAL FROM DUAL;

How to Modify the Sequences

We can modify the sequences using an alter sequence.

SQL> ALTER SEQUENCE tech_test1 INCREMENT BY 50;

The changes impact only the future use of the sequence.
You must be the owner or have alter privilege on that sequence
We cannot change “start with” option. For this sequence needs to be dropped and recreated

Sequence changes are also validated

How to drop the sequence

Dropping a view is done using the DROP sequence command.

Drop sequence test_tech  ;

Dictionary Views for seeing the sequence data

sequence details can be queried from the dictionary by querying either USER_SEQUENCES, ALL_ SEQUENCES, or DBA_ SEQUENCES. There are three categories of views

	DBA_% views about sequences	ALL_% views about sequences	USER_% views about sequences
View about sequences information	dba_sequences	all_ sequences	user_ sequences

To list all sequences owned by the current use

select sequence_name from user_sequences;

To list all sequences in a database:

Select owner, sequence_name from dba_sequences;

To list sequences accessible to the current user:

select sequence_name from all_sequences

How to determine the all information about the sequence?

select sequence_name,min_value,max_value,increment_by,last_number
FROM DBA_SEQUENCES

where OWNER = ‘<owner_name>’
and sequence_NAME = ‘<sequence_name>’;

How to retrieve the current value of an oracle sequence without incrementing it

The last_number column displays the next available sequence number if no cache is specified

SELECT last_number   FROM user_sequences WHERE sequence_name = '';

How to extract the sequence definition (DDL statements) from an Oracle database without having to go through a stack of dictionary views

Syntax:

SQL> set long 1000
SQL> set pagesize 0select DBMS_METADATA.GET_DDL('SEQUENCE','<sequence_name>') from DUAL;

How to set the LAST VALUE value in an Oracle Sequence

ALTER SEQUENCE tech_seq_name INCREMENT BY 250;
SELECT tech_seq_name.nextval FROM dual;
ALTER SEQUENCE tech_seq_name INCREMENT BY 1;

How to Reset a Sequence in Oracle

There are many ways.

a) We can drop and recreate the sequence. But this invalidates all dependent objects (triggers/stored procedures etc.)

b) We can reset by these simple steps

Step 1:Find the latest value of the sequence  
Select tech_seq_name.nextval FROM dual;
Step 2: alter the sequence with increment of the negative value of  the latest value
ALTER SEQUENCE tech_seq_name INCREMENT BY -< latest value>  minvalue 0;
Step 3  Do nextval to set it back to zero
SELECT tech_seq_name.nextval FROM dual;
Step 4: Change increment back to 1
ALTER SEQUENCE tech_seq_name  INCREMENT BY 1;
Example
SELECT tech_seq_name.nextval FROM dual;
-----
250
ALTER SEQUENCE tech_seq_name INCREMENT BY -250;
SELECT tech_seq_name.nextval FROM dual;
ALTER SEQUENCE tech_seq_name  INCREMENT BY 1;

You can find more on this link

How to reset the sequence

Impact of caching the sequences

Sequences are cached with the purpose of improving the fetch performance. In RAC, each instance stores the cache values

We can have gaps in sequence when using cache due to the following reasons

Rollback occurs

System crash or instance crash
The sequence is used in another table

There is a new feature in Oracle 12c for sequences

Session sequences

With Oracle Database 12C new keywords SESSION, GLOBAL are available that can be specified during a sequence creation
CREATE SEQUENCE test_session_seq START WITH 1 INCREMENT BY 1 SESSION;
CREATE SEQUENCE test_global_seq START WITH 1 INCREMENT BY 1 GLOBAL;

Global

Session

creates standard sequence well known in previous release. This is the default.

creates new type session sequence, which is a special type of sequence that is specifically designed to be used with global temporary tables that have session visibility. Session sequence returns a unique range of sequence numbers only within a session, but not across sessions. Another difference is that session sequences are not persistent. If a session goes away, so does the state of the session sequences that were accessed during the session.

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