How to create a table that can query multiple remote clusters

Question

How do I create a table that can query other clusters or instances?

Answer

Below is a simple example to test functionality.

In this example, ClickHouse Cloud is use but the example will work when using self-hosted clusters also. The targets will need to change to the urls/hosts/dns of a target node or load balancer.

In cluster A:

./clickhouse client --host clusterA.us-west-2.aws.clickhouse.cloud --secure --password 'Password123!'

Create the database:

create database db1;

Create the table:

 CREATE TABLE db1.table1_remote1
(
    `id` UInt32,
    `timestamp_column` DateTime,
    `string_column` String
)
ENGINE = MergeTree()
ORDER BY id;

Insert some sample rows:

insert into db1.table1_remote1
values
(1, '2023-09-29 00:01:00', 'a'),
(2, '2023-09-29 00:02:00', 'b'),
(3, '2023-09-29 00:03:00', 'c');

In cluster B:

./clickhouse client --host clusterB.us-east-2.aws.clickhouse.cloud --secure --password 'Password123!'

Create the database:

create database db1;

Create the table:

CREATE TABLE db1.table1_remote2
(
    `id` UInt32,
    `timestamp_column` DateTime,
    `string_column` String
)
ENGINE = MergeTree()
ORDER BY id;

Insert sample rows:

insert into db1.table1_remote1
values
(4, '2023-09-29 00:04:00', 'x'),
(5, '2023-09-29 00:05:00', 'y'),
(6, '2023-09-29 00:06:00', 'z');

In Cluster C:
*this cluster will be used to gather the data from the other two clusters, however, can also be used as a source.

./clickhouse client --host clusterC.us-west-2.aws.clickhouse.cloud --secure --password 'Password123!'

Create the database:

create database db1;

Create the remote tables with remoteSecure() to connect to the other clusters.
Definition for remote cluster A table:

CREATE TABLE db1.table1_remote1_main
(
    `id` UInt32,
    `timestamp_column` DateTime,
    `string_column` String
) AS remoteSecure('clusterA.us-west-2.aws.clickhouse.cloud:9440', 'db1.table1_remote1', 'default', 'Password123!');

Definition for remote cluster B table:

CREATE TABLE db1.table1_remote2_main
(
    `id` UInt32,
    `timestamp_column` DateTime,
    `string_column` String
) AS remoteSecure('clusterB.us-east-2.aws.clickhouse.cloud:9440', 'db1.table1_remote2', 'default', 'Password123!')

Create the merge table to be used to gather results:

create table db1.table1_merge_remote
(
  id UInt32,
  timestamp_column DateTime,
  string_column String
)
engine = Merge('db1', 'table.\_main');

Test the results:

clickhouse-cloud :) select * from db1.table1_merge_remote;

SELECT *
FROM db1.table1_merge_remote

Query id: 46b6e741-bbd1-47ed-b40e-69ddb6e0c364

┌─id─┬────timestamp_column─┬─string_column─┐
│  1 │ 2023-09-29 00:01:00 │ a             │
│  2 │ 2023-09-29 00:02:00 │ b             │
│  3 │ 2023-09-29 00:03:00 │ c             │
└────┴─────────────────────┴───────────────┘
┌─id─┬────timestamp_column─┬─string_column─┐
│  4 │ 2023-09-29 00:04:00 │ x             │
│  5 │ 2023-09-29 00:05:00 │ y             │
│  6 │ 2023-09-29 00:06:00 │ z             │
└────┴─────────────────────┴───────────────┘

6 rows in set. Elapsed: 0.275 sec. 

For more info:
https://clickhouse.com/docs/en/sql-reference/table-functions/remote
https://clickhouse.com/docs/en/engines/table-engines/special/merge

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