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MySQL

The MySQL engine allows you to perform SELECT and INSERT queries on data that is stored on a remote MySQL server.

Creating a Table​

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [TTL expr1],
name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [TTL expr2],
...
) ENGINE = MySQL('host:port', 'database', 'table', 'user', 'password'[, replace_query, 'on_duplicate_clause'])
SETTINGS
[connection_pool_size=16, ]
[connection_max_tries=3, ]
[connection_wait_timeout=5, ] /* 0 -- do not wait */
[connection_auto_close=true ]
;

See a detailed description of the CREATE TABLE query.

The table structure can differ from the original MySQL table structure:

  • Column names should be the same as in the original MySQL table, but you can use just some of these columns and in any order.
  • Column types may differ from those in the original MySQL table. ClickHouse tries to cast values to the ClickHouse data types.
  • The external_table_functions_use_nulls setting defines how to handle Nullable columns. Default value: 1. If 0, the table function does not make Nullable columns and inserts default values instead of nulls. This is also applicable for NULL values inside arrays.

Engine Parameters

  • host:port β€” MySQL server address.

  • database β€” Remote database name.

  • table β€” Remote table name.

  • user β€” MySQL user.

  • password β€” User password.

  • replace_query β€” Flag that converts INSERT INTO queries to REPLACE INTO. If replace_query=1, the query is substituted.

  • on_duplicate_clause β€” The ON DUPLICATE KEY on_duplicate_clause expression that is added to the INSERT query.

    Example: INSERT INTO t (c1,c2) VALUES ('a', 2) ON DUPLICATE KEY UPDATE c2 = c2 + 1, where on_duplicate_clause is UPDATE c2 = c2 + 1. See the MySQL documentation to find which on_duplicate_clause you can use with the ON DUPLICATE KEY clause.

    To specify on_duplicate_clause you need to pass 0 to the replace_query parameter. If you simultaneously pass replace_query = 1 and on_duplicate_clause, ClickHouse generates an exception.

Simple WHERE clauses such as =, !=, >, >=, <, <= are executed on the MySQL server.

The rest of the conditions and the LIMIT sampling constraint are executed in ClickHouse only after the query to MySQL finishes.

Supports multiple replicas that must be listed by |. For example:

CREATE TABLE test_replicas (id UInt32, name String, age UInt32, money UInt32) ENGINE = MySQL(`mysql{2|3|4}:3306`, 'clickhouse', 'test_replicas', 'root', 'clickhouse');

Usage Example​

Table in MySQL:

mysql> CREATE TABLE `test`.`test` (
-> `int_id` INT NOT NULL AUTO_INCREMENT,
-> `int_nullable` INT NULL DEFAULT NULL,
-> `float` FLOAT NOT NULL,
-> `float_nullable` FLOAT NULL DEFAULT NULL,
-> PRIMARY KEY (`int_id`));
Query OK, 0 rows affected (0,09 sec)

mysql> insert into test (`int_id`, `float`) VALUES (1,2);
Query OK, 1 row affected (0,00 sec)

mysql> select * from test;
+------+----------+-----+----------+
| int_id | int_nullable | float | float_nullable |
+------+----------+-----+----------+
| 1 | NULL | 2 | NULL |
+------+----------+-----+----------+
1 row in set (0,00 sec)

Table in ClickHouse, retrieving data from the MySQL table created above:

CREATE TABLE mysql_table
(
`float_nullable` Nullable(Float32),
`int_id` Int32
)
ENGINE = MySQL('localhost:3306', 'test', 'test', 'bayonet', '123')
SELECT * FROM mysql_table
β”Œβ”€float_nullable─┬─int_id─┐
β”‚ ᴺᡁᴸᴸ β”‚ 1 β”‚
β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”˜

Settings​

Default settings are not very efficient, since they do not even reuse connections. These settings allow you to increase the number of queries run by the server per second.

connection_auto_close​

Allows to automatically close the connection after query execution, i.e. disable connection reuse.

Possible values:

  • 1 β€” Auto-close connection is allowed, so the connection reuse is disabled
  • 0 β€” Auto-close connection is not allowed, so the connection reuse is enabled

Default value: 1.

connection_max_tries​

Sets the number of retries for pool with failover.

Possible values:

  • Positive integer.
  • 0 β€” There are no retries for pool with failover.

Default value: 3.

connection_pool_size​

Size of connection pool (if all connections are in use, the query will wait until some connection will be freed).

Possible values:

  • Positive integer.

Default value: 16.

See Also​

Original article