MariaDB starting with 10.3
CREATE SEQUENCE is being introduced in MariaDB 10.3. Details may still change before 10.3 is released as beta.
CREATE [OR REPLACE] [TEMPORARY] SEQUENCE [IF NOT EXISTS] sequence_name [ INCREMENT [ BY | = ] increment ] [ MINVALUE [=] minvalue | NO MINVALUE | NOMINVALUE ] [ MAXVALUE [=] maxvalue | NO MAXVALUE | NOMAXVALUE ] [ START [ WITH | = ] start ] [ CACHE [=] cache ] [ [ NO ] CYCLE ] [table_options]
table_options can be any of the normal table options in CREATE TABLE but the most usable ones are
NOMAXVALUE and NOMINVALUE are there to allow one to create
SEQUENCE's using the Oracle syntax.
To get the next value from a sequence, use
NEXT VALUE FOR sequence
or in Oracle mode (SQL_MODE=ORACLE)
For retrieving the last value used by the current connection from a sequence use:
PREVIOUS VALUE FOR sequence_name
or in Oracle mode (SQL_MODE=ORACLE)
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
CACHE) 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().
DROP SEQUENCE can be used to drop a sequence,
Arguments to create
The following options may be used:
|INCREMENT||1||Increment to use for values. May be negative|
|MINVALUE||1 if INCREMENT > 0 and -9223372036854775807 if INCREMENT < 0||Minimum value for the sequence|
|MAXVALUE||9223372036854775806 if INCREMENT > 0 and -1 if INCREMENT < 0||Max value for sequence|
|START||MINVALUE if INCREMENT > 0 and MAX_VALUE if INCREMENT< 0||First value that the sequence will generate|
|CACHE||1000||Number of values that should be cached. 0 if no CACHE. The underlying table will be updated first time a new sequence number is generated and each time the cache runs out.|
|CYCLE||0 (= NO CYCLE)||1 if the sequence should start again from MINVAL after it has run out of values|
Constraints on create arguments
To be able to create a legal sequence, the following must hold:
- MAXVALUE >= start
- MAXVALUE > MINVALUE
- START >= MINVALUE
- MAXVALUE <= 9223372036854775806 (LONGLONG_MAX-1)
- MINVALUE >= -9223372036854775807 (LONGLONG_MIN+1)
Note that sequences can't generate the maximum/minimum 64 bit number because of the constraint of
If one wants to use
SEQUENCE in a master-master setup or with Galera one
INCREMENT=0. This will tell the
SEQUENCE to use auto_increment_increment and auto_increment_offset to generate unique values for each server.
MariaDB 10.3 supports both ANSI SQL and Oracle syntax for sequences.
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.
One of the goals with the
SEQUENCE implementation is that all old
tools, like 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 mysqldump to work but also gives the additional benefit that one can change all properties of a sequence with a single insert. New applications can of course also use
- Updates to the sequence table will change the single row.
- Doing a select on the sequence shows the current state of the sequence, except the values that are reserved in the cache. The column
next_valueshows 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
SEQUENCEobject. In effect, this will discard the cached values.
- A lot of normal table operations work on
SEQUENCEtables. See next section.
Table operations that work with
- SHOW CREATE TABLE sequence_name. This shows the table structure that is behind the
SEQUENCEincluding 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
- SHOW TABLES
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 `t1` ( `next_value` bigint(21) NOT NULL COMMENT 'next not cached value', `min_value` bigint(21) NOT NULL COMMENT 'min value', `max_value` bigint(21) NOT NULL COMMENT 'max value', `start` bigint(21) NOT NULL COMMENT 'start value', `increment` bigint(21) NOT NULL COMMENT 'increment value', `cache` bigint(21) NOT NULL COMMENT 'cache size', `cycle` tinyint(1) unsigned NOT NULL COMMENT 'cycle state', `round` bigint(21) NOT NULL COMMENT 'How many cycles has been done' ) ENGINE=InnoDB SEQUENCE=1
MariaDB [test]> select * from t1\G
next_value: 1 min_value: 1 max_value: 9223372036854775806 start: 1 increment: 1 cache: 1000 cycle: 0 round: 0
round column is incremented every time the sequence wraps around.
Thanks to Jianwe Zhao from Aliyun for his work on SEQUENCE in AliSQL, which gave ideas and inspiration for this work.