ClickHouse/docs/en/table_functions/remote.rst

remote
------

``remote('addresses_expr', db, table[, 'user'[, 'password']])``

or  

``remote('addresses_expr', db.table[, 'user'[, 'password']])``

- Allows accessing a remote server without creating a Distributed table.

``addresses_expr`` - An expression that generates addresses of remote servers.

This may be just one server address. The server address is host:port, or just the host. The host can be specified as the server name, or as the IPv4 or IPv6 address. An IPv6 address is specified in square brackets. The port is the TCP port on the remote server. If the port is omitted, it uses tcp_port from the server's config file (by default, 9000).

Note: As an exception, when specifying an IPv6 address, the port is required.

Examples:

.. code-block:: text

  example01-01-1
  example01-01-1:9000
  localhost
  127.0.0.1
  [::]:9000
  [2a02:6b8:0:1111::11]:9000

Multiple addresses can be comma-separated. In this case, the query goes to all the specified addresses (like to shards with different data) and uses distributed processing.

Example:

.. code-block:: text

  example01-01-1,example01-02-1

Part of the expression can be specified in curly brackets. The previous example can be written as follows:

.. code-block:: text

  example01-0{1,2}-1

Curly brackets can contain a range of numbers separated by two dots (non-negative integers). In this case, the range is expanded to a set of values that generate shard addresses. If the first number starts with zero, the values are formed with the same zero alignment.
The previous example can be written as follows:

.. code-block:: text

  example01-{01..02}-1

If you have multiple pairs of curly brackets, it generates the direct product of the corresponding sets.

Addresses and fragments in curly brackets can be separated by the pipe (|) symbol. In this case, the corresponding sets of addresses are interpreted as replicas, and the query will be sent to the first healthy replica. The replicas are evaluated in the order currently set in the 'load_balancing' setting.

Example:

.. code-block:: text

  example01-{01..02}-{1|2}

This example specifies two shards that each have two replicas.

The number of addresses generated is limited by a constant. Right now this is 1000 addresses.

Using the 'remote' table function is less optimal than creating a Distributed table, because in this case, the server connection is re-established for every request. In addition, if host names are set, the names are resolved, and errors are not counted when working with various replicas. When processing a large number of queries, always create the Distributed table ahead of time, and don't use the 'remote' table function.

The 'remote' table function can be useful in the following cases:
 * Accessing a specific server for data comparison, debugging, and testing.
 * Queries between various ClickHouse clusters for research purposes.
 * Infrequent distributed requests that are made manually.
 * Distributed requests where the set of servers is re-defined each time.

The username can be omitted. In this case, the 'default' username is used.
The password can be omitted. In this case, an empty password is used.