The Ultimate Guide to High Availability with MariaDB
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Sequence Overview

This page is about sequence objects. For details about the storage engine, see Sequence Storage Engine.

Introduction

A sequence is an object that generates a sequence of numeric values, as specified by the CREATE SEQUENCE statement.

CREATE SEQUENCE will create a sequence that generates new values when called with NEXT VALUE FOR sequence_name. It's an alternative to AUTO INCREMENT when one wants to have more control of how the numbers are generated. As the SEQUENCE caches values (up to the CACHE value in the CREATE SEQUENCE statement, by default 1000) it can in some cases be much faster than AUTO INCREMENT. Another benefit is that one can access the last value generated by all used sequences, which solves one of the limitations with LAST_INSERT_ID().

Creating a Sequence

The CREATE SEQUENCE statement is used to create a sequence. Here is an example of a sequence starting at 100, incrementing by 10 each time:

CREATE SEQUENCE s START WITH 100 INCREMENT BY 10;

The CREATE SEQUENCE statement, along with defaults, can be viewd with the SHOW CREATE SEQUENCE STATEMENT, for example:

SHOW CREATE SEQUENCE s\G
*************************** 1. row ***************************
       Table: s
Create Table: CREATE SEQUENCE `s` start with 100 minvalue 1 maxvalue 9223372036854775806 
  increment by 10 cache 1000 nocycle ENGINE=InnoDB

Using Sequence Objects

To get the next value from a sequence, use

NEXT VALUE FOR sequence_name

or

NEXTVAL(sequence_name)

or in Oracle mode (SQL_MODE=ORACLE)

sequence_name.nextval

For retrieving the last value used by the current connection from a sequence use:

PREVIOUS VALUE FOR sequence_name

or

LASTVAL(sequence_name)

or in Oracle mode (SQL_MODE=ORACLE)

sequence_name.currval

For example:

SELECT NEXTVAL(s);
+------------+
| NEXTVAL(s) |
+------------+
|        100 |
+------------+

SELECT NEXTVAL(s);
+------------+
| NEXTVAL(s) |
+------------+
|        110 |
+------------+

SELECT LASTVAL(s);
+------------+
| LASTVAL(s) |
+------------+
|        110 |
+------------+

Using Sequences in DEFAULT

Sequences can be used in DEFAULT:

create sequence s1;
create table t1 (a int primary key default (next value for s1), b int);
insert into t1 (b) values (1),(2);
select * from t1;
+---+------+
| a | b    |
+---+------+
| 1 |    1 |
| 2 |    2 |
+---+------+

Changing a Sequence

The ALTER SEQUENCE statement is used for changing sequences. For example, to restart the sequence at another value:

ALTER SEQUENCE s RESTART 50;

SELECT NEXTVAL(s);
+------------+
| NEXTVAL(s) |
+------------+
|         50 |
+------------+

The SETVAL function can also be used to set the next value to be returned for a SEQUENCE, for example:

SELECT SETVAL(s, 100);
+----------------+
| SETVAL(s, 100) |
+----------------+
|            100 |
+----------------+

SETVAL can only be used to increase the sequence value. Attempting to set a lower value will fail, returning NULL:

SELECT SETVAL(s, 50);
+---------------+
| SETVAL(s, 50) |
+---------------+
|          NULL |
+---------------+

Dropping a Sequence

The DROP SEQUENCE statement is used to drop a sequence, for example:

DROP SEQUENCE s;

Replication

If one wants to use Sequences in a master-master setup or with Galera one should use INCREMENT=0. This will tell the Sequence to use auto_increment_increment and auto_increment_offset to generate unique values for each server.

Standards Compliance

MariaDB supports both ANSI SQL and Oracle syntax for sequences.

However as SEQUENCE is implemented as a special kind of table, it uses the same namespace as tables. The benefits are that sequences show up in SHOW TABLES, and one can also create a sequence with CREATE TABLE and drop it with DROP TABLE. One can SELECT from it as from any other table. This ensures that all old tools that work with tables should work with sequences.

Since sequence objects act as regular tables in many contexts, they will be affected by LOCK TABLES. This is not the case in other DBMS, such as Oracle, where LOCK TABLE does not affect sequences.

Notes

One of the goals with the Sequence implementation is that all old tools, such as mariadb-dump (previously mysqldump), should work unchanged, while still keeping the normal usage of sequence standard compatibly.

To make this possible, sequence is currently implemented as a table with a few exclusive properties.

The special properties for sequence tables are:

  • A sequence table has always one row.
  • When one creates a sequence, either with CREATE TABLE or CREATE SEQUENCE, one row will be inserted.
  • If one tries to insert into a sequence table, the single row will be updated. This allows mariadb-dump to work but also gives the additional benefit that one can change all properties of a sequence with a single insert. New applications should of course also use ALTER SEQUENCE.
  • UPDATE or DELETE can't be performed on Sequence objects.
  • Doing a select on the sequence shows the current state of the sequence, except the values that are reserved in the cache. The next_value column shows the next value not reserved by the cache.
  • FLUSH TABLES will close the sequence and the next sequence number generated will be according to what's stored in the Sequence object. In effect, this will discard the cached values.
  • A number of normal table operations work on Sequence tables. See next section.

Table Operations that Work with Sequences

Implementation

Internally, sequence tables are created as a normal table without rollback (the InnoDB, Aria and MySAM engines support this), wrapped by a sequence engine object. This allowed us to create sequences with almost no performance impact for normal tables. (The cost is one 'if' per insert if the binary log is enabled).

Underlying Table Structure

The following example shows the table structure of sequences and how it can be used as a table. (Output of results are slightly edited to make them easier to read)

create sequence t1;
show create sequence t1\G
*************************** 1. row ***************************
  CREATE SEQUENCE `t1` start with 1 minvalue 1 maxvalue 9223372036854775806
  increment by 1 cache 1000 nocycle ENGINE=InnoDB

show create table t1\G
*************************** 1. row ***************************
Create Table: CREATE TABLE `t1` (
  `next_not_cached_value` bigint(21) NOT NULL,
  `minimum_value` bigint(21) NOT NULL,
  `maximum_value` bigint(21) NOT NULL,
  `start_value` bigint(21) NOT NULL COMMENT 'start value when sequences is created or value if RESTART is used',
  `increment` bigint(21) NOT NULL COMMENT 'increment value',
  `cache_size` bigint(21) unsigned NOT NULL,
  `cycle_option` tinyint(1) unsigned NOT NULL COMMENT '0 if no cycles are allowed, 1 if the sequence should begin a new cycle when maximum_value is passed',
  `cycle_count` bigint(21) NOT NULL COMMENT 'How many cycles have been done'
) ENGINE=InnoDB SEQUENCE=1


select * from t1\G
next_not_cached_value: 1
 minimum_value: 1
 maximum_value: 9223372036854775806
  start_value: 1
  increment: 1
  cache_size: 1000
  cycle_option: 0
  cycle_count: 0

The cycle_count column is incremented every time the sequence wraps around.

Credits

  • Thanks to Jianwe Zhao from Aliyun for his work on SEQUENCE in AliSQL, which gave ideas and inspiration for this work.
  • Thanks to Peter Gulutzan,who helped test and gave useful comments about the implementation.

See Also

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