CREATE SEQUENCE

Synopsis

Use the CREATE SEQUENCE statement to create a new sequence in the current schema.

Syntax

• Grammar
• Diagram

create_sequence ::= CREATE SEQUENCE [ IF NOT EXISTS ] sequence_name 
                    sequence_options

sequence_name ::= '<Text Literal>'

sequence_options ::= [ INCREMENT [ BY ] increment ] 
                     [ MINVALUE minvalue | NO MINVALUE ] 
                     [ MAXVALUE maxvalue | NO MAXVALUE ] 
                     [ START [ WITH ] start ] [ CACHE cache ]

Semantics

create_sequence

CREATE SEQUENCE sequence_name [ IF NOT EXISTS ]

Specify the name of the sequence (sequence_name). An error is raised if a sequence with that name already exists in the current schema and IF NOT EXISTS is not specified.

sequence_options

INCREMENT BY increment

Specify the difference between consecutive values in the sequence. Default is 1.

MINVALUE minvalue | NO MINVALUE

Specify the minimum value allowed in the sequence. If this value is reached (in a sequence with a negative increment), nextval() will return an error. If NO MINVALUE is specified, the default value will be used. Default is 1.

MAXVALUE maxvalue | NO MAXVALUE

Specify the maximum value allowed in the sequence. If this value is reached, nextval() will return an error. If NO MAXVALUE is specified, the default will be used. Default is 2<sup>63</sup> - 1.

START WITH start

Specify the first value in the sequence. start cannot be less than minvalue. Default is 1.

CACHE cache

Specify how many numbers from the sequence to cache in the client. Default is 1.

Cache

In YSQL as in PostgreSQL, the sequence’s data is stored in a persistent system table. In YSQL this table has one row per sequence and it stores the sequence data in two values:

last_val

Stores the last value used or the next value to be used.

is_called

Stores whether last_val has been used. If false, last_val is the next value in the sequence. Otherwise, last_val + INCREMENT is the next one.

By default (when INCREMENT is 1), each call to nextval() updates last_val for that sequence. In YSQL, the table holding the sequence’s data is replicated as opposed to being in the local file system. Each update to this table requires two RPCs (and will be optimized to one RPC in the future), In any case, the latency experienced by a call to nextval() in YSQL will be significantly higher than the same operation in Postgres. To avoid such performance degradation, we recommend using a cache value with a value large enough. Cached values are stored in the memory of the local node, and retrieving such values avoids any RPCs, so the latency of one cache allocation can be amortized over all the numbers allocated for the cache.

SERIAL types create a sequence with a cache with default value of 1. So SERIAL types should be avoided, and their equivalent statement should be used. Instead of creating a table with a SERIAL type like this:

CREATE TABLE t(k SERIAL)

You should create a sequence with a large enough cache first, and then set the column that you want to have a serial type to DEFAULT to nextval() of the sequence.

CREATE SEQUENCE t_k_seq CACHE 10000;
CREATE TABLE t(k integer NOT NULL DEFAULT nextval('t_k_seq'));

Examples

Create a simple sequence that increments by 1 every time nextval() is called.

postgres=# CREATE SEQUENCE s;

CREATE SEQUENCE

Call nextval().

postgres=# SELECT nextval('s');

 nextval
---------
       1
(1 row)

Create a sequence with a cache of 10,000 values.

postgres=# CREATE SEQUENCE s2 CACHE 10000;

CREATE SEQUENCE

In the same session, select nextval().

SELECT nextval('s2');

 nextval
---------
       1
(1 row)

In a different session, select nextval().

SELECT nextval('s2');

nextval
---------
   10001
(1 row)

Create a sequence that starts at 0. MINVALUE also has to be changed from its default 1 to something less than or equal to 0.

CREATE SEQUENCE s3 START 0 MINVALUE 0;

CREATE SEQUENCE

SELECT nextval('s3');

nextval
---------
       0
(1 row)

See also

• DROP SEQUENCE
• currval()
• lastval()
• nextval()