The Clickhouse plugin for dbt (data build tool)
Project description
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-core dbt-clickhouse
NOTE: Beginning in v1.8, dbt-core and adapters are decoupled. Therefore, the installation mentioned above explicitly includes both dbt-core and the desired adapter.If you use a version prior to 1.8.0 the pip installation command should look like this:
pip install dbt-clickhouse
Supported features
- Table materialization
- View materialization
- Incremental materialization
- Materialized View materializations (uses the
TO
form of MATERIALIZED VIEW, experimental) - Seeds
- Sources
- Docs generate
- Tests
- Snapshots
- Most dbt-utils macros (now included in dbt-core)
- Ephemeral materialization
- Distributed table materialization (experimental)
- Distributed incremental materialization (experimental)
- 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.
local_db_prefix: [<empty string>] # Database prefix of local tables on shards for distributed materializations. If empty, it uses the same database as the distributed table.
allow_automatic_deduplication: [False] # Enable ClickHouse automatic deduplication for Replicated tables
tcp_keepalive: [False] # Native client only, specify TCP keepalive configuration. Specify custom keepalive settings as [idle_time_sec, interval_sec, probes].
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
Option | Description | Default if any |
---|---|---|
engine | The table engine (type of table) to use when creating tables | MergeTree() |
order_by | A tuple of column names or arbitrary expressions. This allows you to create a small sparse index that helps find data faster. | tuple() |
partition_by | A partition is a logical combination of records in a table by a specified criterion. The partition key can be any expression from the table columns. | |
sharding_key | Sharding key determines the destination server when inserting into distributed engine table. The sharding key can be random or as an output of a hash function | rand() ) |
primary_key | Like order_by, a ClickHouse primary key expression. If not specified, ClickHouse will use the order by expression as the primary key | |
unique_key | A tuple of column names that uniquely identify rows. Used with incremental models for updates. | |
inserts_only | If set to True for an incremental model, incremental updates will be inserted directly to the target table without creating intermediate table. It has been deprecated in favor of the append incremental strategy , which operates in the same way. If inserts_only is set, incremental_strategy is ignored. |
|
incremental_strategy | Incremental model update strategy: delete+insert , append , or insert_overwrite . See the following Incremental Model Strategies |
default |
incremental_predicates | Additional conditions to be applied to the incremental materialization (only applied to delete+insert strategy |
|
settings | A map/dictionary of "TABLE" settings to be used to DDL statements like 'CREATE TABLE' with this model | |
query_settings | A map/dictionary of ClickHouse user level settings to be used with INSERT or DELETE statements in conjunction with this model |
|
ttl | A TTL expression to be used with the table. The TTL expression is a string that can be used to specify the TTL for the table. |
Column Configuration
Option | Description | Default if any |
---|---|---|
codec | A string consisting of arguments passed to CODEC() in the column's DDL. For example: codec: "Delta, ZSTD" will be interpreted as CODEC(Delta, ZSTD) . |
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 setuse_lw_deletes=true
in your profile (which will enable that setting for your dbt sessions) - Lightweight deletes are now production ready, but there may be performance and other problems on ClickHouse versions earlier than 23.3.
- 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.
The insert_overwrite Strategy (Experimental)
[IMPORTANT]
Currently, the insert_overwrite strategy is not fully functional with distributed materializations.
Performs the following steps:
- Create a staging (temporary) table with the same structure as the incremental model relation:
CREATE TABLE <staging> AS <target>
. - Insert only new records (produced by
SELECT
) into the staging table. - Replace only new partitions (present in the staging table) into the target table.
This approach has the following advantages:
- It is faster than the default strategy because it doesn't copy the entire table.
- It is safer than other strategies because it doesn't modify the original table until the INSERT operation completes successfully: in case of intermediate failure, the original table is not modified.
- It implements "partitions immutability" data engineering best practice. Which simplifies incremental and parallel data processing, rollbacks, etc.
The strategy requires partition_by
to be set in the model configuration. Ignores all other strategies-specific parameters of the model config.
Additional ClickHouse Macros
Model Materialization Utility Macros
The following macros are included to facilitate creating ClickHouse specific tables and views:
engine_clause
-- Uses theengine
model configuration property to assign a ClickHouse table engine. dbt-clickhouse uses theMergeTree
engine by default.partition_cols
-- Uses thepartition_by
model configuration property to assign a ClickHouse partition key. No partition key is assigned by default.order_cols
-- Uses theorder_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 unsortedprimary_key_clause
-- Uses theprimary_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 thecluster
profile property to add anON CLUSTER
clause to certain dbt-operations: distributed materializations, views creation, database creation.ttl_config
-- Uses thettl
model configuration property to assign a ClickHouse table TTL expression. No TTL is assigned by default.
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:
Argument Name | Description |
---|---|
bucket | The bucket base url, such as https://datasets-documentation.s3.eu-west-3.amazonaws.com/nyc-taxi . https:// is assumed if no protocol is provided. |
path | The S3 path to use for the table query, such as /trips_4.gz . S3 wildcards are supported. |
fmt | The expected ClickHouse input format (such as TSV or CSVWithNames ) of the referenced S3 objects. |
structure | The column structure of the data in bucket, as a list of name/datatype pairs, such as ['id UInt32', 'date DateTime', 'value String'] If not provided ClickHouse will infer the structure. |
aws_access_key_id | The S3 access key id. |
aws_secret_access_key | The S3 secret key. |
role_arn | The ARN of a ClickhouseAccess IAM role to use to securely access the S3 objects. See this documentation for more information. |
compression | The compression method used with the S3 objects. If not provided ClickHouse will attempt to determine compression based on the file name. |
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.
Dictionary materializations (experimental)
See the tests in https://github.com/ClickHouse/dbt-clickhouse/blob/main/tests/integration/adapter/dictionary/test_dictionary.py for examples of how to implement materializations for ClickHouse dictionaries
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:
- Creates temp view with sql query to get right structure
- Create empty local tables based on view
- Create distributed table based on local tables.
- 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.
- The Append Strategy just insert data into distributed table.
- The Delete+Insert Strategy creates distributed temp table to work with all data on every shard.
- 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));
Contributing
We welcome contributions from the community to help improve the dbt-ClickHouse adapter. Whether you’re fixing a bug, adding a new feature, or enhancing documentation, your efforts are greatly appreciated!
Please take a moment to read our Contribution Guide to get started. The guide provides detailed instructions on setting up your environment, running tests, and submitting pull requests.
Original Author
ClickHouse wants to thank @silentsokolov for creating this connector and for their valuable contributions.
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