ClickHouse

ClickHouse is a fast open-source column-oriented database management system that allows generating analytical data reports in real-time using SQL queries.

The ClickHouse Wrapper allows you to read and write data from ClickHouse within your Postgres database.

Warning

Restoring a logical backup of a database with a materialized view using a foreign table can fail. For this reason, either do not use foreign tables in materialized views or use them in databases with physical backups enabled.

Supported Data Types

Postgres Type	ClickHouse Type
boolean	UInt8
smallint	Int16
integer	UInt16
integer	Int32
bigint	UInt32
bigint	Int64
bigint	UInt64
real	Float32
double precision	Float64
text	String
date	Date
timestamp	DateTime

Preparation

Before you get started, make sure the wrappers extension is installed on your database:

create extension if not exists wrappers with schema extensions;

and then create the foreign data wrapper:

create foreign data wrapper clickhouse_wrapper
  handler click_house_fdw_handler
  validator click_house_fdw_validator;

Secure your credentials (optional)

By default, Postgres stores FDW credentials inide pg_catalog.pg_foreign_server in plain text. Anyone with access to this table will be able to view these credentials. Wrappers is designed to work with Vault, which provides an additional level of security for storing credentials. We recommend using Vault to store your credentials.

-- Save your ClickHouse credential in Vault and retrieve the `key_id`
insert into vault.secrets (name, secret)
values (
  'clickhouse',
  'tcp://default:@localhost:9000/default'
)
returning key_id;

Connecting to ClickHouse

We need to provide Postgres with the credentials to connect to ClickHouse, and any additional options. We can do this using the create server command:

With VaultWithout Vault

create server clickhouse_server
  foreign data wrapper clickhouse_wrapper
  options (
    conn_string_id '<key_ID>' -- The Key ID from above.
  );

create server clickhouse_server
  foreign data wrapper clickhouse_wrapper
  options (
    conn_string 'tcp://default:@localhost:9000/default'
  );

Some connection string examples:

• tcp://user:password@host:9000/clicks?compression=lz4&ping_timeout=42ms
• tcp://default:PASSWORD@abc.eu-west-1.aws.clickhouse.cloud:9440/default?connection_timeout=30s&ping_before_query=false&secure=true

Check more connection string parameters.

Creating Foreign Tables

The ClickHouse Wrapper supports data reads and writes from ClickHouse.

Integration	Select	Insert	Update	Delete	Truncate
ClickHouse	✅	✅	✅	✅	❌

For example:

create foreign table my_clickhouse_table (
  id bigint,
  name text
)
  server clickhouse_server
  options (
    table 'people'
  );

Foreign table options

The full list of foreign table options are below:

• table - Source table name in ClickHouse, required.

This can also be a subquery enclosed in parentheses, for example,

table '(select * from my_table)'

Parametrized view is also supported in the subquery. In this case, you need to define a column for each parameter and use where to pass values to them. For example,

 create foreign table test_vw (
   id bigint,
   col1 text,
   col2 bigint,
   _param1 text,
   _param2 bigint
 )
   server clickhouse_server
   options (
     table '(select * from my_view(column1=${_param1}, column2=${_param2}))'
   );

 select * from test_vw where _param1='aaa' and _param2=32;

• rowid_column - Primary key column name, optional for data scan, required for data modify

Query Pushdown Support

This FDW supports where, order by and limit clause pushdown, as well as parametrized view (see above).

Examples

Some examples on how to use ClickHouse foreign tables.

Basic example

This will create a "foreign table" inside your Postgres database called people:

-- Run below SQLs on ClickHouse to create source table
drop table if exists people;
create table people (
    id Int64,
    name String
)
engine=MergeTree()
order by id;

-- Add some test data
insert into people values (1, 'Luke Skywalker'), (2, 'Leia Organa'), (3, 'Han Solo');

Create foreign table on Postgres database:

create foreign table people (
  id bigint,
  name text
)
  server clickhouse_server
  options (
    table 'people'
  );

-- data scan
select * from people;

-- data modify
insert into people values (4, 'Yoda');
update people set name = 'Princess Leia' where id = 2;
delete from people where id = 3;