In MySQL Cluster NDB 7.5.2 and later, the table comment in a CREATE TABLE or ALTER TABLE statement can also be used to specify an NDB_TABLE option, which consists of one or more name-value pairs, separated by commas if need be, following the string NDB_TABLE=. Complete syntax for names and values syntax is shown here:

COMMENT="NDB_TABLE=ndb_table_option[,ndb_table_option[,...]]"

ndb_table_option:
    NOLOGGING={1|0}
  | READ_BACKUP={1|0}
  | FRAGMENT_COUNT_TYPE={ONE_PER_NODE|ONE_PER_NODE_GROUP|ONE_PER_LDM|ONE_PER_LDM_PER_NODE}
  | FULLY_REPLICATED={1|0}

Spaces are not permitted within the quoted string. The string is case-insensitive.

The four NDB table options that can be set as part of a comment in this way are described in more detail in the next few paragraphs.

NOLOGGING: Using 1 corresponds to having ndb_table_no_logging enabled, but has no actual effect. Provided as a placeholder, mostly for completeness of ALTER TABLE statements.

READ_BACKUP: Setting this option to 1 has the same effect as though ndb_read_backup were enabled; enables reading from any replica. Setting this option to 1 automatically sets FRAGMENT_COUNT_TYPE to ONE_PER_LDM_PER_NODE_GROUP (see next item). Starting with MySQL Cluster NDB 7.5.3, you can set READ_BACKUP for an existing table online, using an ALTER TABLE statement similar to one of those shown here:

ALTER TABLE ... ALGORITHM=INPLACE, COMMENT="NDB_TABLE=READ_BACKUP=1";

ALTER TABLE ... ALGORITHM=INPLACE, COMMENT="NDB_TABLE=READ_BACKUP=0";

(See Bug #80858, Bug #23001617). For more information about the ALGORITHM option for ALTER TABLE, see Section 14.1.8.2, “ALTER TABLE Online Operations in MySQL Cluster”.

FRAGMENT_COUNT_TYPE: Provides additional control over assignment and placement of partitions. The following four schemes are supported:

FULLY_REPLICATED controls whether the table is fully replicated, that is, whether each data node has a complete copy of the table. To enable full replication of the table, use FULLY_REPLICATED=1. You must also set (or have already set) the table's FRAGMENT_COUNT_TYPE to either one of ONE_PER_NODE_GROUP or ONE_PER_LDM_PRE_NODE_GROUP in order for this to work.

This setting can also be controlled using the ndb_fully_replicated system variable. Setting it to ON enables the option by default for all new NDB tables; the default is OFF, which maintains the previous behavior (as in MySQL Cluster NDB 7.5.1 and earlier, before support for fully replicated tables was introduced). The ndb_data_node_neighbour system variable is also used for fully replicated tables, to ensure that when a fully replicated table is accessed, we access the data node which is local to this MySQL Server.

An example of a CREATE TABLE statement using such a comment when creating an NDB table is shown here:

mysql> CREATE TABLE t1 (
     >     c1 INT NOT NULL AUTO_INCREMENT PRIMARY KEY, 
     >     c2 VARCHAR(100), 
     >     c3 VARCHAR(100) ) 
     > ENGINE=NDB
     > COMMENT="NDB_TABLE=READ_BACKUP=0,FRAGMENT_COUNT_TYPE=ONE_PER_NODE";  

The comment is displayed as part of the ouput of SHOW CREATE TABLE. The text of the comment is also available from querying the MySQL Information Schema TABLES table, as in this example:

mysql> SELECT TABLE_NAME, TABLE_SCHEMA, TABLE_COMMENT 
     > FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME="t1";
+------------+--------------+----------------------------------------------------------+
| TABLE_NAME | TABLE_SCHEMA | TABLE_COMMENT                                            |
+------------+--------------+----------------------------------------------------------+
| t1         | c            | NDB_TABLE=READ_BACKUP=0,FRAGMENT_COUNT_TYPE=ONE_PER_NODE |
| t1         | d            |                                                          |
+------------+--------------+----------------------------------------------------------+
2 rows in set (0.00 sec)

This comment syntax is also supported with ALTER TABLE statements for NDB tables. Keep in mind that a table comment used with ALTER TABLE replaces any existing comment which the table might have.

mysql> ALTER TABLE t1 COMMENT="NDB_TABLE=FRAGMENT_COUNT_TYPE=ONE_PER_NODE_GROUP";
Query OK, 0 rows affected (0.40 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT TABLE_NAME, TABLE_SCHEMA, TABLE_COMMENT 
     > FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME="t1";
+------------+--------------+--------------------------------------------------+
| TABLE_NAME | TABLE_SCHEMA | TABLE_COMMENT                                    |
+------------+--------------+--------------------------------------------------+
| t1         | c            | NDB_TABLE=FRAGMENT_COUNT_TYPE=ONE_PER_NODE_GROUP |
| t1         | d            |                                                  |
+------------+--------------+--------------------------------------------------+
2 rows in set (0.01 sec)

Because the READ_BACKUP value was not carried over to the new comment set by the ALTER TABLE statement, there is no longer a way to retrieve the value previously set for it. To avoid this from problem, it is suggested that you preserve any such values from the existing comment string, like this:

mysql> SELECT TABLE_NAME, TABLE_SCHEMA, TABLE_COMMENT 
     > FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME="t1";
+------------+--------------+----------------------------------------------------------+
| TABLE_NAME | TABLE_SCHEMA | TABLE_COMMENT                                            |
+------------+--------------+----------------------------------------------------------+
| t1         | c            | NDB_TABLE=READ_BACKUP=0,FRAGMENT_COUNT_TYPE=ONE_PER_NODE |
| t1         | d            |                                                          |
+------------+--------------+----------------------------------------------------------+
2 rows in set (0.00 sec)

mysql> ALTER TABLE t1 COMMENT="NDB_TABLE=READ_BACKUP=0,FRAGMENT_COUNT_TYPE=ONE_PER_NODE_GROUP";
Query OK, 0 rows affected (1.56 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT TABLE_NAME, TABLE_SCHEMA, TABLE_COMMENT 
     > FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME="t1";
+------------+--------------+----------------------------------------------------------------+
| TABLE_NAME | TABLE_SCHEMA | TABLE_COMMENT                                                  |
+------------+--------------+----------------------------------------------------------------+
| t1         | c            | NDB_TABLE=READ_BACKUP=0,FRAGMENT_COUNT_TYPE=ONE_PER_NODE_GROUP |
| t1         | d            |                                                                |
+------------+--------------+----------------------------------------------------------------+
2 rows in set (0.01 sec)