5.6. Limitations

There are some limitations in Mroonga storage engine.

5.6.1. Limitations of table

A table has the following limitations. This limitation is derived from Groonga.

• The maximum one key size: 4KiB

• The maximum total size of keys: 4GiB

• The maximum number of records:

  • No primary key table: 1,073,741,815 (2 ³⁰ - 9)

  • PRIMARY KEY or PRIMARY KEY USING BTREE table: 1,073,741,823 (2 ³⁰ - 1)

  • PRIMARY KEY USING HASH table: 536,870,912 (2 ²⁹)

Keep in mind that these limitations may vary depending on conditions.

5.6.2. Limitations of indexing

A full-text index has the following limitations. This limitation is derived from groonga and applied to each table.

• The maximum number of distinct terms: 268,435,455 (more than 268 million)

• The maximum index size: 256GiB

Keep in mind that these limitations may vary depending on conditions.

Confirm by SHOW TABLE STATUS or SHOW INDEX FROM ... whether your table data matches to this limitations.

5.6.3. Limitations about the value of columns

There is a limitation about the value of column in storage mode.

Mroonga storage engine executes automatic conversion against the value NULL.

For example, if the value NULL is used in DATE or DATETIME columns, Mroonga storage engine automatically converts 0 into 1 as the value of month or date.

Thus, the value 0 is treated as the 1st month (January) of the year or the 1st date of the month.

And more, the value NULL is treated as the value of UNIX time 0 (1970-01-01 00:00:00).

This kind of automatic conversion is not restricted to only DATE or DATETIME types.

The value NULL is converted into the default value of columns. In most cases, it will be converted into empty string for column which belongs to type of string, 0 for column which belongs to type of numeric.

Here is an example to show behavior described above.

CREATE TABLE date_limitation (
  id INT PRIMARY KEY AUTO_INCREMENT,
  input varchar(32) DEFAULT NULL,
  date DATE DEFAULT NULL
) ENGINE=mroonga DEFAULT CHARSET=UTF8;
CREATE TABLE datetime_limitation (
  id INT PRIMARY KEY AUTO_INCREMENT,
  input varchar(32) DEFAULT NULL,
  datetime DATETIME DEFAULT NULL
) ENGINE=mroonga DEFAULT CHARSET=UTF8;

-- Test data for date_limitation
INSERT INTO date_limitation (input) VALUES ("NULL");
INSERT INTO date_limitation (input, date) VALUES ("1970-00-00", "1970-00-00");

-- Test data for datetime_limitation
INSERT INTO datetime_limitation (input) VALUES ("NULL");
INSERT INTO datetime_limitation (input, datetime) VALUES ("1970-00-00 00:00:00", "1970-00-00 00:00:00");

Here is the results of execution example:

mysql> select * from date_limitation;
+----+------------+------------+
| id | input      | date       |
+----+------------+------------+
|  1 | NULL       | 1970-01-01 |
|  2 | 1970-00-00 | 1970-01-01 |
+----+------------+------------+
2 rows in set (0.00 sec)

mysql> select * from datetime_limitation;
+----+---------------------+---------------------+
| id | input               | datetime            |
+----+---------------------+---------------------+
|  1 | NULL                | 1970-01-01 00:00:00 |
|  2 | 1970-00-00 00:00:00 | 1970-01-01 00:00:00 |
+----+---------------------+---------------------+
2 rows in set (0.00 sec)

5.6.4. Limitations of column size

A column has the following limitation.

• The maximum stored data size of a column: 256GiB