17.1.6.2. Limits and Differences of MySQL Cluster from Standard MySQL Limits

In this section, we list limits found in MySQL Cluster that either differ from limits found in, or that are not found in, standard MySQL.

Memory usage and recovery. Memory consumed when data is inserted into an NDB table is not automatically recovered when deleted, as it is with other storage engines. Instead, the following rules hold true:

A DELETE statement on an NDB table makes the memory formerly used by the deleted rows available for re-use by inserts on the same table only. However, this memory can be made available for general re-use by performing OPTIMIZE TABLE.

A rolling restart of the cluster also frees any memory used by deleted rows. See Section 17.5.5, "Performing a Rolling Restart of a MySQL Cluster".
A DROP TABLE or TRUNCATE TABLE operation on an NDB table frees the memory that was used by this table for re-use by any NDB table, either by the same table or by another NDB table.

Note

Recall that TRUNCATE TABLE drops and re-creates the table. See Section 13.1.33, "TRUNCATE TABLE Syntax".
Limits imposed by the cluster's configuration. A number of hard limits exist which are configurable, but available main memory in the cluster sets limits. See the complete list of configuration parameters in Section 17.3.2, "MySQL Cluster Configuration Files". Most configuration parameters can be upgraded online. These hard limits include:
- Database memory size and index memory size (DataMemory and IndexMemory, respectively).
  
  DataMemory is allocated as 32KB pages. As each DataMemory page is used, it is assigned to a specific table; once allocated, this memory cannot be freed except by dropping the table.
  
  See Section 17.3.2.6, "Defining MySQL Cluster Data Nodes", for more information.
- The maximum number of operations that can be performed per transaction is set using the configuration parameters MaxNoOfConcurrentOperations and MaxNoOfLocalOperations.
  
  Note
  
  Bulk loading, TRUNCATE TABLE, and ALTER TABLE are handled as special cases by running multiple transactions, and so are not subject to this limitation.
- Different limits related to tables and indexes. For example, the maximum number of ordered indexes in the cluster is determined by MaxNoOfOrderedIndexes, and the maximum number of ordered indexes per table is 16.
Node and data object maximums. The following limits apply to numbers of cluster nodes and metadata objects:
- The maximum number of data nodes is 48.
  
  A data node must have a node ID in the range of 1 to 49, inclusive. (Management and API nodes may use node IDs in the range 1 to 255, inclusive.)
- The total maximum number of nodes in a MySQL Cluster is 255. This number includes all SQL nodes (MySQL Servers), API nodes (applications accessing the cluster other than MySQL servers), data nodes, and management servers.
- The maximum number of metadata objects in current versions of MySQL Cluster is 20320. This limit is hard-coded.
See Section 17.1.6.11, "Previous MySQL Cluster Issues Resolved in MySQL Cluster NDB 7.3", for more information.