17.3.4.3. MySQL Cluster System Variables

have_ndbcluster

YES if mysqld supports NDBCLUSTER tables. DISABLED if --skip-ndbcluster is used.

This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.

multi_range_count

The maximum number of ranges to send to a table handler at once during range selects. The default value is 256. Sending multiple ranges to a handler at once can improve the performance of certain selects dramatically. This is especially true for the NDBCLUSTER table handler, which needs to send the range requests to all nodes. Sending a batch of those requests at once reduces communication costs significantly.

This variable was removed in MySQL 5.6.7.

ndb_autoincrement_prefetch_sz

Determines the probability of gaps in an autoincremented column. Set it to 1 to minimize this. Setting it to a high value for optimization makes inserts faster, but decreases the likelihood that consecutive autoincrement numbers will be used in a batch of inserts. The mininum and default value is 1. The maximum value for ndb_autoincrement_prefetch_sz is 65536.

This variable affects only the number of AUTO_INCREMENT IDs that are fetched between statements; within a given statement, at least 32 IDs are obtained at a time. The default value for ndb_autoincrement_prefetch_sz is 1, to increase the speed of statements inserting single rows.

ndb_cache_check_time

The number of milliseconds that elapse between checks of MySQL Cluster SQL nodes by the MySQL query cache. Setting this to 0 (the default and minimum value) means that the query cache checks for validation on every query.

The recommended maximum value for this variable is 1000, which means that the check is performed once per second. A larger value means that the check is performed and possibly invalidated due to updates on different SQL nodes less often. It is generally not desirable to set this to a value greater than 2000.

ndb_deferred_constraints

Controls whether or not constraint checks are deferred, where these are supported. OFF is the default.

This variable is not normally needed for operation of MySQL Cluster or MySQL Cluster Replication, and is intended primarily for use in testing.

ndb_distribution

Controls the default distribution method for NDB tables. Can be set to either of KEYHASH (key hashing) or LINHASH (linear hashing). KEYHASH is the default.

ndb_extra_logging

This variable can be used to enable recording in the MySQL error log of information specific to the NDB storage engine. It is normally of interest only when debugging NDB storage engine code.

The default value is 0, which means that the only NDB-specific information written to the MySQL error log relates to transaction handling. If the value is greater than 0 but less than 10, NDB table schema and connection events are also logged, as well as whether or not conflict resolution is in use, and other NDB errors and information. If the value is set to 10 or more, information about NDB internals, such as the progress of data distribution among cluster nodes, is also written to the MySQL error log.

ndb_force_send

Forces sending of buffers to NDB immediately, without waiting for other threads. Defaults to ON.

ndb_join_pushdown

This variable controls whether joins on NDB tables are pushed down to the NDB kernel (data nodes). Previously, a join was handled using multiple accesses of NDB by the SQL node; however, when ndb_join_pushdown is enabled, a pushable join is sent in its entirety to the data nodes, where it can be distributed among the data nodes and executed in parallel on multiple copies of the data, with a single, merged result being returned to mysqld. This can reduce greatly the number of round trips between an SQL node and the data nodes required to handle such a join.

By default, ndb_join_pushdown is enabled.

In order for a join to be pushable, it must meet the following conditions:

You can see whether a given join can be pushed down by checking it with EXPLAIN; when the join can be pushed down, you can see references to the pushed join in the Extra column of the output, as shown in this example:

mysql> EXPLAIN    ->     SELECT e.first_name, e.last_name, t.title, d.dept_name    ->         FROM employees e    ->         JOIN dept_emp de ON e.emp_no=de.emp_no    ->         JOIN departments d ON d.dept_no=de.dept_no    ->         JOIN titles t ON e.emp_no=t.emp_no\G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: d         type: ALLpossible_keys: PRIMARY          key: NULL      key_len: NULL          ref: NULL         rows: 9        Extra: Parent of 4 pushed join@1*************************** 2. row ***************************           id: 1  select_type: SIMPLE        table: de         type: refpossible_keys: PRIMARY,emp_no,dept_no          key: dept_no      key_len: 4          ref: employees.d.dept_no         rows: 5305        Extra: Child of 'd' in pushed join@1*************************** 3. row ***************************           id: 1  select_type: SIMPLE        table: e         type: eq_refpossible_keys: PRIMARY          key: PRIMARY      key_len: 4          ref: employees.de.emp_no         rows: 1        Extra: Child of 'de' in pushed join@1*************************** 4. row ***************************           id: 1  select_type: SIMPLE        table: t         type: refpossible_keys: PRIMARY,emp_no          key: emp_no      key_len: 4          ref: employees.de.emp_no         rows: 19        Extra: Child of 'e' in pushed join@14 rows in set (0.00 sec)

Two additional sources of information about pushed join performance are available:

ndb_log_apply_status

A read-only variable which shows whether the server was started with the --ndb-log-apply-status option.

ndb_log_bin

Causes updates to NDB tables to be written to the binary log. Setting this variable has no effect if binary logging is not already enabled for the server using log_bin. ndb_log_bin defaults to 1 (ON); normally, there is never any need to change this value in a production environment.

ndb_log_binlog_index

Causes a mapping of epochs to positions in the binary log to be inserted into the ndb_binlog_index table. Setting this variable has no effect if binary logging is not already enabled for the server using log_bin. (In addition, ndb_log_bin must not be disabled.) ndb_log_binlog_index defaults to 1 (ON); normally, there is never any need to change this value in a production environment.

ndb_log_transaction_id

This read-only, Boolean system variable shows whether a slave mysqld writes NDB transaction IDs in the binary log (required to use "active-active" MySQL Cluster Replication with NDB$EPOCH_TRANS() conflict detection). To change the setting, use the --ndb-log-transaction-id option.

ndb_log_transaction_id is not supported in mainline MySQL Server 5.6.

For more information, see Section 17.6.11, "MySQL Cluster Replication Conflict Resolution".

ndb_optimized_node_selection

There are two forms of optimized node selection, described here:

This option takes one of the integer values 0, 1, 2, or 3. 3 is the default. These values affect node selection as follows:

ndb_recv_thread_activation_threshold

When this number of concurrently active threads is reached, the receive thread takes over polling of the cluster connection.

This variable is global in scope. It can also be set on startup using the --ndb-recv-thread-activation-threshold option.

ndb_recv_thread_cpu_mask

CPU mask for locking receiver threads to specific CPUs. This is specified as a hexadecimal bitmask; for example, 0x33 means that one CPU is used per receiver thread. An empty string is the default; setting ndb_recv_thread_cpu_mask to this value removes any receiver thread locks previously set.

This variable is global in scope. It can also be set on startup using the --ndb-recv-thread-cpu-mask option.

ndb_report_thresh_binlog_epoch_slip

This is a threshold on the number of epochs to be behind before reporting binary log status. For example, a value of 3 (the default) means that if the difference between which epoch has been received from the storage nodes and which epoch has been applied to the binary log is 3 or more, a status message will be sent to the cluster log.

ndb_report_thresh_binlog_mem_usage

This is a threshold on the percentage of free memory remaining before reporting binary log status. For example, a value of 10 (the default) means that if the amount of available memory for receiving binary log data from the data nodes falls below 10%, a status message will be sent to the cluster log.

ndb_table_no_logging

When this variable is set to ON or 1, it causes NDB tables not to be checkpointed to disk. More specifically, this setting applies to tables which are created or altered using ENGINE NDB when ndb_table_no_logging is enabled, and continues to apply for the lifetime of the table, even if ndb_table_no_logging is later changed. Suppose that A, B, C, and D are tables that we create (and perhaps also alter), and that we also change the setting for ndb_table_no_logging as shown here:

SET @@ndb_table_no_logging = 1;CREATE TABLE A ... ENGINE NDB;CREATE TABLE B ... ENGINE MYISAM;CREATE TABLE C ... ENGINE MYISAM;ALTER TABLE B ENGINE NDB;SET @@ndb_table_no_logging = 0;CREATE TABLE D ... ENGINE NDB;ALTER TABLE C ENGINE NDB;SET @@ndb_table_no_logging = 1;

After the previous sequence of events, tables A and B are not checkpointed; A was created with ENGINE NDB and B was altered to use NDB, both while ndb_table_no_logging was enabled. However, tables C and D are logged; C was altered to use NDB and D was created using ENGINE NDB, both while ndb_table_no_logging was disabled. Setting ndb_table_no_logging back to 1 or ON does not cause table C or D to be checkpointed.

ndb_table_temporary

When set to ON or 1, this variable causes NDB tables not to be written to disk: This means that no table schema files are created, and that the tables are not logged.

ndb_use_copying_alter_table

Forces NDB to use copying of tables in the event of problems with online ALTER TABLE operations. The default value is OFF.

ndb_use_exact_count

Forces NDB to use a count of records during SELECT COUNT(*) query planning to speed up this type of query. The default value is ON. For faster queries overall, disable this feature by setting the value of ndb_use_exact_count to OFF.

ndb_use_transactions

You can disable NDB transaction support by setting this variable's values to OFF (not recommended). The default is ON.

transaction_allow_batching

When set to 1 or ON, this variable enables batching of statements within the same transaction. To use this variable, autocommit must first be disabled by setting it to 0 or OFF; otherwise, setting transaction_allow_batching has no effect.

It is safe to use this variable with transactions that performs writes only, as having it enabled can lead to reads from the "before" image. You should ensure that any pending transactions are committed (using an explicit COMMIT if desired) before issuing a SELECT.

This variable is currently supported for MySQL Cluster only.

ndbinfo_database

Shows the name used for the NDB information database; the default is ndbinfo. This is a read-only variable whose value is determined at compile time; you can set it by starting the server using --ndbinfo-database=name, which sets the value shown for this variable but does not actually change the name used for the NDB information database.

ndbinfo_max_bytes

Used in testing and debugging only.

ndbinfo_max_rows

Used in testing and debugging only.

ndbinfo_show_hidden

Whether or not the ndbinfo database's underlying internal tables are shown in the mysql client. The default is OFF.

ndbinfo_table_prefix

The prefix used in naming the ndbinfo database's base tables (normally hidden, unless exposed by setting ndbinfo_show_hidden). This is a read-only variable whose default value is "ndb$". You can start the server with the --ndbinfo-table-prefix option, but this merely sets the variable and does not change the actual prefix used to name the hidden base tables; the prefix itself is determined at compile time.

ndbinfo_version

Shows the version of the ndbinfo engine in use; read-only.

ndb_log_empty_epochs

When this variable is set to 0, epoch transactions with no changes are not written to the binary log, although a row is still written even for an empty epoch in ndb_binlog_index.

server_id_bits

The effective value of server_id if the server was started with the --server-id-bits option set to a nondefault value.

If the value of server_id greater than or equal to 2 to the power of server_id_bits, mysqld refuses to start.

This system variable is supported only by MySQL Cluster. server_id_bits is not supported by the standard MySQL Server.