BLES. 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
-----------------------------------------

* SHOW CREATE TABLE sequence_name. This shows the table structure that is
behind the SEQUENCE including the field names that can be used with SELECT or
even CREATE TABLE.
* CREATE TABLE sequence-structure ... SEQUENCE=1
* ALTER TABLE sequence RENAME TO sequence2
* RENAME TABLE sequence_name TO new_sequence_name
* DROP TABLE sequence_name. This is allowed mainly to get old tools like
mariadb-dump to work with sequence tables.
* SHOW TABLES

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.

URL: https://mariadb.com/kb/en/sequence-overview/