dbt-clickhouse

This plugin ports dbt functionality to Clickhouse.

The plugin uses syntax that requires ClickHouse version 22.1 or newer. We do not test older versions of Clickhouse. We also do not currently test Replicated tables or the related ON CLUSTER functionality.

Installation

Use your favorite Python package manager to install the app from PyPI, e.g.

pip install dbt-clickhouse

Supported features

• [x] Table materialization
• [x] View materialization
• [x] Incremental materialization
• [x] Materialized View materializations (uses the TO form of MATERIALIZED VIEW, experimental)
• [x] Seeds
• [x] Sources
• [x] Docs generate
• [x] Tests
• [x] Snapshots
• [x] Most dbt-utils macros (now included in dbt-core)
• [x] Ephemeral materialization
• [x] Distributed table materialization (experimental)
• [x] Distributed incremental materialization (experimental)
• [x] Contracts

Usage Notes

SET Statement Warning

In many environments, using the SET statement to persist a ClickHouse setting across all DBT queries is not reliable and can cause unexpected failures. This is particularly true when using HTTP connections through a load balancer that distributes queries across multiple nodes (such as ClickHouse cloud), although in some circumstances this can also happen with native ClickHouse connections. Accordingly, we recommend configuring any required ClickHouse settings in the "custom_settings" property of the DBT profile as a best practice, instead of relying on a prehook "SET" statement as has been occasionally suggested.

Database

The dbt model relation identifier database.schema.table is not compatible with Clickhouse because Clickhouse does not support a schema. So we use a simplified approach schema.table, where schema is the Clickhouse database. Using the default database is not recommended.

Example Profile

Default values are in brackets:

your_profile_name:
  target: dev
  outputs:
    dev:
      type: clickhouse
      schema: [default] # ClickHouse database for dbt models

      # optional
      driver: [http] # http or native.  If not set this will be autodetermined based on port setting
      host: [localhost] 
      port: [8123]  # If not set, defaults to 8123, 8443, 9000, 9440 depending on the secure and driver settings 
      user: [default] # User for all database operations
      password: [<empty string>] # Password for the user
      cluster: [<empty string>] If set, certain DDL/table operations will be executed with the `ON CLUSTER` clause using this cluster. Distributed materializations require this setting to work. See the following ClickHouse Cluster section for more details.
      verify: [True] # Validate TLS certificate if using TLS/SSL
      secure: [False] # Use TLS (native protocol) or HTTPS (http protocol)
      retries: [1] # Number of times to retry a "retriable" database exception (such as a 503 'Service Unavailable' error)
      compression: [<empty string>] Use gzip compression if truthy (http), or compression type for a native connection
      connect_timeout: [10] # Timeout in seconds to establish a connection to ClickHouse
      send_receive_timeout: [300] # Timeout in seconds to receive data from the ClickHouse server
      cluster_mode: [False] # Use specific settings designed to improve operation on Replicated databases (recommended for ClickHouse Cloud)
      use_lw_deletes: [False] Use the strategy `delete+insert` as the default incremental strategy.
      check_exchange: [True] # Validate that clickhouse support the atomic EXCHANGE TABLES command.  (Not needed for most ClickHouse versions)
      local_suffix [_local] # Table suffix of local tables on shards for distributed materializations.
      allow_automatic_deduplication [False] # Enable ClickHouse automatic deduplication for Replicated tables
      custom_settings: [{}] # A dictionary/mapping of custom ClickHouse settings for the connection - default is empty.
      
      # Native (clickhouse-driver) connection settings
      sync_request_timeout: [5] Timeout for server ping
      compress_block_size: [1048576] Compression block size if compression is enabled
      

Model Configuration

ClickHouse Cluster

The cluster setting in profile enables dbt-clickhouse to run against a ClickHouse cluster.

Effective Scope

if cluster is set in profile, on_cluster_clause now will return cluster info for:

• Database creation
• View materialization
• Distributed materializations
• Models with Replicated engines

table and incremental materializations with non-replicated engine will not be affected by cluster setting (model would be created on the connected node only).

Compatibility

If a model has been created without a cluster setting, dbt-clickhouse will detect the situation and run all DDL/DML without on cluster clause for this model.

A Note on Model Settings

ClickHouse has several types/levels of "settings". In the model configuration above, two types of these are configurable. settings means the SETTINGS clause used in CREATE TABLE/VIEW types of DDL statements, so this is generally settings that are specific to the specific ClickHouse table engine. The new query_settings is use to add a SETTINGS clause to the INSERT and DELETE queries used for model materialization (including incremental materializations). There are hundreds of ClickHouse settings, and it's not always clear which is a "table" setting and which is a "user" setting (although the latter are generally available in the system.settings table.) In general the defaults are recommended, and any use of these properties should be carefully researched and tested.

Known Limitations

• Ephemeral models/CTEs don't work if placed before the "INSERT INTO" in a ClickHouse insert statement, see https://github.com/ClickHouse/ClickHouse/issues/30323. This should not affect most models, but care should be taken where an ephemeral model is placed in model definitions and other SQL statements.

Incremental Model Strategies

As of version 1.3.2, dbt-clickhouse supports three incremental model strategies.

The Default (Legacy) Strategy

Historically ClickHouse has had only limited support for updates and deletes, in the form of asynchronous "mutations." To emulate expected dbt behavior, dbt-clickhouse by default creates a new temporary table containing all unaffected (not deleted, not changed) "old" records, plus any new or updated records, and then swaps or exchanges this temporary table with the existing incremental model relation. This is the only strategy that preserves the original relation if something goes wrong before the operation completes; however, since it involves a full copy of the original table, it can be quite expensive and slow to execute.

The Delete+Insert Strategy

ClickHouse added "lightweight deletes" as an experimental feature in version 22.8. Lightweight deletes are significantly faster than ALTER TABLE ... DELETE operations, because they don't require rewriting ClickHouse data parts. The incremental strategy delete+insert utilizes lightweight deletes to implement incremental materializations that perform significantly better than the "legacy" strategy. However, there are important caveats to using this strategy:

• Lightweight deletes must be enabled on your ClickHouse server using the setting allow_experimental_lightweight_delete=1 or you must set use_lw_deletes=true in your profile (which will enable that setting for your dbt sessions)
• As suggested by the setting name, lightweight delete functionality is still experimental and there are still known issues that must be resolved before the feature is considered production ready, so usage should be limited to datasets that are easily recreated
• This strategy operates directly on the affected table/relation (with creating any intermediate or temporary tables), so if there is an issue during the operation, the data in the incremental model is likely to be in an invalid state
• When using lightweight deletes, dbt-clickhouse enabled the setting allow_nondeterministic_mutations. In some very rare cases using non-deterministic incremental_predicates this could result in a race condition for the updated/deleted items (and related log messages in the ClickHouse logs). To ensure consistent results the incremental predicates should only include sub-queries on data that will not be modified during the incremental materialization.

The Append Strategy

This strategy replaces the inserts_only setting in previous versions of dbt-clickhouse. This approach simply appends new rows to the existing relation. As a result duplicate rows are not eliminated, and there is no temporary or intermediate table. It is the fastest approach if duplicates are either permitted in the data or excluded by the incremental query WHERE clause/filter.

Additional ClickHouse Macros

Model Materialization Utility Macros

The following macros are included to facilitate creating ClickHouse specific tables and views:

• engine_clause -- Uses the engine model configuration property to assign a ClickHouse table engine. dbt-clickhouse uses the MergeTree engine by default.
• partition_cols -- Uses the partition_by model configuration property to assign a ClickHouse partition key. No partition key is assigned by default.
• order_cols -- Uses the order_by model configuration to assign a ClickHouse order by/sorting key. If not specified ClickHouse will use an empty tuple() and the table will be unsorted
• primary_key_clause -- Uses the primary_key model configuration property to assign a ClickHouse primary key. By default, primary key is set and ClickHouse will use the order by clause as the primary key.
• on_cluster_clause -- Uses the cluster profile property to add an ON CLUSTER clause to certain dbt-operations: distributed materializations, views creation, database creation.

s3Source Helper Macro

The s3source macro simplifies the process of selecting ClickHouse data directly from S3 using the ClickHouse S3 table function. It works by populating the S3 table function parameters from a named configuration dictionary (the name of the dictionary must end in s3). The macro first looks for the dictionary in the profile vars, and then in the model configuration. The dictionary can contain any of the following keys used to populate the parameters of the S3 table function:

See the S3 test file for examples of how to use this macro.

Contracts and Constraints

Only exact column type contracts are supported. For example, a contract with a UInt32 column type will fail if the model returns a UInt64 or other integer type. ClickHouse also support only CHECK constraints on the entire table/model. Primary key, foreign key, unique, and column level CHECK constraints are not supported. (See ClickHouse documentation on primary/order by keys.)

Materialized Views (Experimental)

A materialized_view materialization should be a SELECT from an existing (source) table. The adapter will create a target table with the model name and a ClickHouse MATERIALIZED VIEW with the name <model_name>_mv. Unlike PostgreSQL, a ClickHouse materialized view is not "static" (and has no corresponding REFRESH operation). Instead, it acts as an "insert trigger", and will insert new rows into the target table using the defined SELECT "transformation" in the view definition on rows inserted into the source table. See the [test file] (https://github.com/ClickHouse/dbt-clickhouse/blob/main/tests/integration/adapter/materialized_view/test_materialized_view.py) for an introductory example of how to use this functionality.

Distributed materializations

Notes:

• dbt-clickhouse queries now automatically include the setting insert_distributed_sync = 1 in order to ensure that downstream incremental materialization operations execute correctly. This could cause some distributed table inserts to run more slowly than expected.

Distributed table materialization

Distributed table created with following steps:

1. Creates temp view with sql query to get right structure
2. Create empty local tables based on view
3. Create distributed table based on local tables.
4. Data inserts into distributed table, so it is distributed across shards without duplicating.

Distributed table model example

{{
    config(
        materialized='distributed_table',
        order_by='id, created_at',
        sharding_key='cityHash64(id)',
        engine='ReplacingMergeTree'
    )
}}

select id, created_at, item from {{ source('db', 'table') }}

Generated migrations

CREATE TABLE db.table_local on cluster cluster
(
    `id` UInt64,
    `created_at` DateTime,
    `item` String
)
ENGINE = ReplacingMergeTree
ORDER BY (id, created_at)
SETTINGS index_granularity = 8192;


CREATE TABLE db.table on cluster cluster
(
    `id` UInt64,
    `created_at` DateTime,
    `item` String
)
ENGINE = Distributed('cluster', 'db', 'table_local', cityHash64(id));

Distributed incremental materialization

Incremental model based on the same idea as distributed table, the main difficulty is to process all incremental strategies correctly.

1. The Append Strategy just insert data into distributed table.
2. The Delete+Insert Strategy creates distributed temp table to work with all data on every shard.
3. The Default (Legacy) Strategy creates distributed temp and intermediate tables for the same reason.

Only shard tables are replacing, because distributed table does not keep data. The distributed table reloads only when the full_refresh mode is enabled or the table structure may have changed.

Distributed incremental model example

{{
    config(
        materialized='distributed_incremental',
        engine='MergeTree',
        incremental_strategy='append',
        unique_key='id,created_at'
    )
}}

select id, created_at, item from {{ source('db', 'table') }}

Generated migrations

CREATE TABLE db.table_local on cluster cluster
(
    `id` UInt64,
    `created_at` DateTime,
    `item` String
)
ENGINE = MergeTree
SETTINGS index_granularity = 8192;


CREATE TABLE db.table on cluster cluster
(
    `id` UInt64,
    `created_at` DateTime,
    `item` String
)
ENGINE = Distributed('cluster', 'db', 'table_local', cityHash64(id));

Running Tests

This adapter passes all of dbt basic tests as presented in dbt's official docs: https://docs.getdbt.com/docs/contributing/testing-a-new-adapter#testing-your-adapter. Use pytest tests to run tests.

You can customize the test environment via environment variables. We recommend doing so with the pytest pytest-dotenv plugin combined with root level test.env configuration file (this file should not be checked into git). The following environment variables are recognized:

1. DBT_CH_TEST_HOST - Default=localhost
2. DBT_CH_TEST_USER - your ClickHouse username. Default=default
3. DBT_CH_TEST_PASSWORD - your ClickHouse password. Default=''
4. DBT_CH_TEST_PORT - ClickHouse client port. Default=8123 (The default is automatically changed to the correct port if DBT_CH_TEST_USE_DOCKER is enabled)
5. DBT_CH_TEST_DB_ENGINE - Database engine used to create schemas. Defaults to '' (server default)
6. DBT_CH_TEST_USE_DOCKER - Set to True to run clickhouse-server docker image (see tests/docker-compose.yml). Requires docker-compose. Default=False
7. DBT_CH_TEST_CH_VERSION - ClickHouse docker image to use. Defaults to latest
8. DBT_CH_TEST_INCLUDE_S3 - Include S3 tests. Default=False since these are currently dependent on a specific ClickHouse S3 bucket/test dataset
9. DBT_CH_TEST_CLUSTER_MODE - Use the profile value
10. DBT_CH_TEST_CLUSTER - ClickHouse cluster name, if DBT_CH_TEST_USE_DOCKER set to true, only test_replica and test_shard is valid (see tests/test_config.xml for cluster settings)

Original Author

ClickHouse wants to thank @silentsokolov for creating this connector and for their valuable contributions.