CREATE SEQUENCE

Function

CREATE SEQUENCE adds a sequence to the current database. The owner of a sequence is the user who creates the sequence.

Precautions

• A sequence is a special table that stores arithmetic sequence. Such a table is controlled by DBMS. It has no actual meaning and is usually used to generate unique identifiers for rows or tables.
• If a schema name is given, the sequence is created in the specified schema; otherwise, it is created in the current schema. The sequence name must be different from the names of other sequences, tables, indexes, views in the same schema.
• After the sequence is created, functions nextval() and generate_series(1,N) insert data to the table. Make sure that the number of times for invoking nextval is greater than or equal to N+1. Otherwise, errors will be reported because the number of times for invoking function generate_series() is N+1.
• A sequence cannot be created in the template1 database.

Syntax

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CREATE SEQUENCE [ IF NOT EXISTS ] name [ INCREMENT [ BY ] increment ]
    [ MINVALUE minvalue | NO MINVALUE | NOMINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE | NOMAXVALUE] 
    [ START [ WITH ] start ] [ CACHE cache ] [ [ NO ] CYCLE | NOCYCLE ] 
    [ OWNED BY { table_name.column_name | NONE } ];

Parameter Description

• IF NOT EXISTS

  If IF NOT EXISTS is specified and a sequence with the same name does not exist, the sequence can be created successfully. If a sequence with the same name already exists during sequence creation, the system will display a message indicating that the sequence already exists and no further operations will be performed. No error will be reported.

  This parameter is supported only by 9.1.0 and later versions.

• name

  Specifies the name of the sequence to be created.

  Value range: The value can contain only lowercase letters, uppercase letters, special characters #_$, and digits.

• increment

  Specifies the step for a sequence. A positive generates an ascending sequence, and a negative generates a decreasing sequence.

  The default value is 1.

• MINVALUE minvalue | NO MINVALUE| NOMINVALUE

  Specifies the minimum value of the sequence. If MINVALUE is not declared, or NO MINVALUE is declared, the default value of the ascending sequence is 1, and that of the descending sequence is -2⁶³-1.

  NOMINVALUE is equivalent to NO MINVALUE.

• MAXVALUE maxvalue | NO MAXVALUE| NOMAXVALUE

  Specifies the maximum value in a sequence. If MAXVALUE is not declared or NO MAXVALUE is declared, the default value of the ascending sequence is 2⁶³-1, and that of the descending sequence is -1.

  NOMAXVALUE is equivalent to NO MAXVALUE.

• start

  Specifies the start value of the sequence.

  The default value for ascending sequences is minvalue and for descending sequences maxvalue.

• cache

  Specifies the number sequences stored in the memory for quick access purposes. Within a cache period, the CN does not request a sequence number from the GTM. Instead, the CN uses the sequence number that is locally applied for in advance.

  In cluster versions 9.1.0.100 and later, the default value is specified by the GUC parameter default_sequence_cache.

  • In a newly installed cluster of 9.1.0.100 or later, the default value is 20.
  • If the cluster is upgraded to version 9.1.0.100 or later from an earlier version, the default value will be 1. This means that only one value can be generated at a time, and no cache will be available.
  

  • It is not recommended that you define cache, and maxvalue, and minvalue at the same time. The continuity of sequences cannot be ensured after cache is defined because unacknowledged sequences may be generated, wasting sequence number segments.
  • You are advised not to set a large value for cache (less than 100000000). Otherwise, it takes a long time to cache the sequence number (the first NEXTVAL in each cache period). Set a proper value for cache based on services to ensure quick access without wasting sequence numbers.
• CYCLE

  Used to ensure that sequences can recycle after the number of sequences reaches maxvalue or minvalue.

  If you declare NO CYCLE, any invocation of nextval would return an error after the sequence reaches its maximum value.

  NOCYCLE is equivalent to NO CYCLE.

  The default value is NO CYCLE.

  If the sequence is defined as CYCLE, the sequence uniqueness cannot be ensured.

• OWNED BY-

  Associates a sequence with a specified column included in a table. In this way, the sequence will be deleted when you delete its associated field or the table where the field belongs. The associated table and sequence must be owned by the same user and in the same schema. OWNED BY only establishes the association between a table column and the sequence. The sequence is not created for this column.

  If the default value is OWNED BY NONE, indicating that such association does not exist.

  

  You are not advised to use the sequence created using OWNED BY in other tables. If multiple tables need to share a sequence, the sequence must not belong to a specific table.

Examples

Create an ascending sequence named serial, which starts from 101:

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CREATE SEQUENCE serial
 START 101
 CACHE 20;

Select the next number from the sequence:

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SELECT nextval('serial');
 nextval 
 ---------
      101

Select the next number from the sequence:

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SELECT nextval('serial');
 nextval 
 ---------
      102

Create a sequence associated with the table:

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CREATE TABLE customer_address
(
    ca_address_sk             integer               not null,
    ca_address_id             char(16)              not null,
    ca_street_number          char(10)                      ,
    ca_street_name            varchar(60)                   ,
    ca_street_type            char(15)                      ,
    ca_suite_number           char(10)                      ,
    ca_city                   varchar(60)                   ,
    ca_county                 varchar(30)                   ,
    ca_state                  char(2)                       ,
    ca_zip                    char(10)                      ,
    ca_country                varchar(20)                   ,
    ca_gmt_offset             decimal(5,2)                  ,
    ca_location_type          char(20)                     
) ;

CREATE SEQUENCE serial1
 START 101
 CACHE 20
OWNED BY customer_address.ca_address_sk;

Use SERIAL to create a serial table serial_table for primary key auto-increment.

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CREATE TABLE serial_table(a int, b serial);
INSERT INTO serial_table (a) VALUES (1),(2),(3);
SELECT * FROM serial_table ORDER BY b;
 a | b
---+---
 1 | 1
 2 | 2
 3 | 3
(3 rows)

Helpful Links

DROP SEQUENCE ALTER SEQUENCE