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Hooks and operations

Assumed knowledge

Getting started with hooks and operations

Effective database administration sometimes requires additional SQL statements to be run, for example:

  • Creating UDFs
  • Managing row- or column-level permissions
  • Vacuuming tables on Redshift
  • Creating partitions in Redshift Spectrum external tables
  • Resuming/pausing/resizing warehouses in Snowflake
  • Refreshing a pipe in Snowflake
  • Create a share on Snowflake
  • Cloning a database on Snowflake

dbt provides hooks and operations so you can version control and execute these statements as part of your dbt project.

About hooks

Hooks are snippets of SQL that are executed at different times:

  • pre-hook: executed before a model, seed or snapshot is built.
  • post-hook: executed after a model, seed or snapshot is built.
  • on-run-start: executed at the start of dbt build, dbt compile, dbt docs generate, dbt run, dbt seed, dbt snapshot, or dbt test.
  • on-run-end: executed at the end of dbt build, dbt compile, dbt docs generate, dbt run, dbt seed, dbt snapshot, or dbt test.

Hooks are a more-advanced capability that enable you to run custom SQL, and leverage database-specific actions, beyond what dbt makes available out-of-the-box with standard materializations and configurations.

In older versions of dbt, the most common use of post-hook was to execute grant statements, to apply database permissions to models right after creating them. We recommend using the grants resource config instead, in order to automatically apply grants when your dbt model runs.

If (and only if) you can't leverage the grants resource-config, you can use post-hook to perform more advanced workflows:

  • Need to apply grants in a more complex way, which the dbt Core grants config doesn't (yet) support.
  • Need to perform post-processing that dbt does not support out-of-the-box. For example, analyze table, alter table set property, alter table ... add row access policy, etc.

Examples using hooks

You can use hooks to trigger actions at certain times when running an operation or building a model, seed, or snapshot.

For more information about when hooks can be triggered, see reference sections for on-run-start and on-run-end hooks and pre-hooks and post-hooks.

You can use hooks to provide database-specific functionality not available out-of-the-box with dbt. For example, you can use a config block to run an ALTER TABLE statement right after building an individual model using a post-hook:

{{ config(
"alter table {{ this }} ..."
) }}

Calling a macro in a hook

You can also use a macro to bundle up hook logic. Check out some of the examples in the reference sections for on-run-start and on-run-end hooks and pre- and post-hooks.

About operations

Operations are macros that you can run using the run-operation command. As such, operations aren't actually a separate resource in your dbt project — they are just a convenient way to invoke a macro without needing to run a model.

Explicitly execute the SQL in an operation

Unlike hooks, you need to explicitly execute a query within a macro, by using either a statement block or a helper macro like the run_query macro. Otherwise, dbt will return the query as a string without executing it.

This macro performs a similar action as the above hooks:

{% macro grant_select(role) %}
{% set sql %}
grant usage on schema {{ target.schema }} to role {{ role }};
grant select on all tables in schema {{ target.schema }} to role {{ role }};
grant select on all views in schema {{ target.schema }} to role {{ role }};
{% endset %}

{% do run_query(sql) %}
{% do log("Privileges granted", info=True) %}
{% endmacro %}

To invoke this macro as an operation, execute dbt run-operation grant_select --args '{role: reporter}'.

$ dbt run-operation grant_select --args '{role: reporter}'
Running with dbt=0.16.1
Privileges granted

Full usage docs for the run-operation command can be found here.

Additional examples

These examples from the community highlight some of the use-cases for hooks and operations!