A Sequence engine allows the creation of ascending or descending sequences of numbers 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.
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_
TO or seq_
A sequence can go backward too. In this case the final value will always be 1, 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 | +-----+
This engine is particularly used in joins. 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 | +-----+
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.
If the SEQUENCE storage engine is installed, it is not possible to create a table with a name which follows the SEQUENCE pattern:
MariaDB [test]> 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:
MariaDB [test]> ALTER TABLE seq_1_to_100 ENGINE = BLACKHOLE; Query OK, 0 rows affected (0.07 sec) MariaDB [test]> 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:
MariaDB [test]> DROP TABLE seq_1_to_100; Query OK, 0 rows affected (0.00 sec) MariaDB [test]> 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:
MariaDB [test]> CREATE TEMPORARY TABLE seq_1_to_100 (col INT) ENGINE = InnoDB; Query OK, 0 rows affected (0.15 sec) MariaDB [test]> SELECT * FROM seq_1_to_100; Empty set (0.00 sec)