Sequence Storage Engine

A Sequence engine allows the creation of ascending or descending sequences of numbers (positive integers) with a given starting value, ending value and increment.

It creates completely virtual, ephemeral tables automatically when you need them. There is no way to create a Sequence table explicitly. Nor are they ever written to disk or create .frm files. They are read-only, transactional, and support XA.

Installing

Until MariaDB 10.0, the Sequence engine is usually distributed as a dynamic plugin, not part of the server binary. To be able to use it, you need to install it first:

INSTALL SONAME "ha_sequence";

From MariaDB 10.1, the Sequence engine is installed by default.

If it has been correctly installed, SHOW ENGINES will list the Sequence storage engine as supported:

SHOW ENGINES\G
...
*************************** 5. row ***************************
      Engine: MyISAM
     Support: YES
     Comment: MyISAM storage engine
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 6. row ***************************
      Engine: SEQUENCE
     Support: YES
     Comment: Generated tables filled with sequential values
Transactions: YES
          XA: YES
  Savepoints: YES
*************************** 7. row ***************************
      Engine: MRG_MyISAM
     Support: YES
     Comment: Collection of identical MyISAM tables
Transactions: NO
          XA: NO
  Savepoints: NO

...

Usage and Examples

To use a Sequence table, you simply select from it, as in

SELECT * FROM seq_1_to_5;
+-----+
| seq |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
+-----+

To use a sequence in a statement, you select from the table named by a pattern seq_FROM_to_TO or seq_FROM_to_TO_step_STEP.

In the case of an odd step, the sequence will commence with the FROM, and end at the final result before TO.

SELECT * FROM seq_1_to_15_step_3;
+-----+
| seq |
+-----+
|   1 |
|   4 |
|   7 |
|  10 |
|  13 |
+-----+

A sequence can go backwards too. In this case the final value will always be the TO value, so that a descending sequence has the same values as an ascending sequence:

SELECT * FROM seq_5_to_1_step_2;
+-----+
| seq |
+-----+
|   5 |
|   3 |
|   1 |
+-----+

SELECT * FROM seq_15_to_1_step_3;
+-----+
| seq |
+-----+
|  13 |
|  10 |
|   7 |
|   4 |
|   1 |
+-----+

SELECT * FROM seq_15_to_2_step_3;
+-----+
| seq |
+-----+
|  14 |
|  11 |
|   8 |
|   5 |
|   2 |
+-----+

This engine is particularly useful with joins and subqueries. For example, this query finds all prime numbers below 50:

SELECT seq FROM seq_2_to_50 s1 WHERE 0 NOT IN
     (SELECT s1.seq % s2.seq FROM seq_2_to_50 s2 WHERE s2.seq <= sqrt(s1.seq));
+-----+
| seq |
+-----+
|   2 |
|   3 |
|   5 |
|   7 |
|  11 |
|  13 |
|  17 |
|  19 |
|  23 |
|  29 |
|  31 |
|  37 |
|  41 |
|  43 |
|  47 |
+-----+

And almost (without 2, the only even prime number) the same result with joins:

SELECT s1.seq FROM seq_2_to_50 s1 JOIN seq_2_to_50 s2 
  WHERE s1.seq > s2.seq AND s1.seq % s2.seq <> 0 
  GROUP BY s1.seq HAVING s1.seq - COUNT(*) = 2;
+-----+
| seq |
+-----+
|   3 |
|   5 |
|   7 |
|  11 |
|  13 |
|  17 |
|  19 |
|  23 |
|  29 |
|  31 |
|  37 |
|  41 |
|  43 |
|  47 |
+-----+

Sequence tables can also be useful in date calculations. For example, to find the day of the week that a particular date has fallen on over a 40 year period (perhaps for birthday planning ahead!):

SELECT DAYNAME('1980-12-05' + INTERVAL (seq) YEAR) day,
    '1980-12-05' + INTERVAL (seq) YEAR date FROM seq_0_to_40;
+-----------+------------+
| day       | date       |
+-----------+------------+
| Friday    | 1980-12-05 |
| Saturday  | 1981-12-05 |
| Sunday    | 1982-12-05 |
...
| Friday    | 2014-12-05 |
| Saturday  | 2015-12-05 |
| Monday    | 2016-12-05 |
| Tuesday   | 2017-12-05 |
| Wednesday | 2018-12-05 |
| Thursday  | 2019-12-05 |
| Saturday  | 2020-12-05 |
+-----------+------------+

Although Sequence tables can only directly make use of positive integers, they can indirectly be used to return negative results by making use of the CAST statement. For example:

SELECT CAST(seq AS INT) - 5 x FROM seq_5_to_1;
+----+
| x  |
+----+
|  0 |
| -1 |
| -2 |
| -3 |
| -4 |
+----+

CAST is required to avoid a BIGINT UNSIGNED value is out of range error.

Sequence tables, while virtual, are still tables, so they must be in a database. This means that a default database must be selected (for example, via the USE command) to be able to query a Sequence table. The information_schema database cannot be used as the default for a Sequence table.

Table Name Conflicts

If the SEQUENCE storage engine is installed, it is not possible to create a table with a name which follows the SEQUENCE pattern:

CREATE TABLE seq_1_to_100 (col INT) ENGINE = InnoDB;
ERROR 1050 (42S01): Table 'seq_1_to_100' already exists

However, a SEQUENCE table can be converted to another engine and the new table can be referred in any statement:

ALTER TABLE seq_1_to_100 ENGINE = BLACKHOLE;

SELECT * FROM seq_1_to_100;
Empty set (0.00 sec)

While a SEQUENCE table cannot be dropped, it is possible to drop the converted table. The SEQUENCE table with the same name will still exist:

DROP TABLE seq_1_to_100;

SELECT COUNT(*) FROM seq_1_to_100;
+----------+
| COUNT(*) |
+----------+
|      100 |
+----------+
1 row in set (0.00 sec)

A temporary table with a SEQUENCE-like name can always be created and used:

CREATE TEMPORARY TABLE seq_1_to_100 (col INT) ENGINE = InnoDB;

SELECT * FROM seq_1_to_100;
Empty set (0.00 sec)

Resources

• Sometimes its the little things - Dean Ellis tries out the Sequence engine.
• MariaDB’s Sequence Storage Engine - Federico Razzoli writes more in-depth on the engine.