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Defer

Defer is a powerful feature that makes it possible to run a subset of models or tests in a sandbox environment without having to first build their upstream parents. This can save time and computational resources when you want to test a small number of models in a large project.

Use 'defer' to modify end-of-pipeline models by pointing to production models, instead of running everything upstream.Use 'defer' to modify end-of-pipeline models by pointing to production models, instead of running everything upstream.

Defer requires that a manifest from a previous dbt invocation be passed to the --state flag or env var. Together with the state: selection method, these features enable "Slim CI". Read more about state.

An alternative command that accomplishes similar functionality for different use cases is dbt clone - see the docs for clone for more information.

It is possible to use separate state for state:modified and --defer, by passing paths to different manifests to each of the --state/DBT_STATE and --defer-state/DBT_DEFER_STATE. This enables more granular control in cases where you want to compare against logical state from one environment or past point in time, and defer to applied state from a different environment or point in time. If --defer-state is not specified, deferral will use the manifest supplied to --state. In most cases, you will want to use the same state for both: compare logical changes against production, and also "fail over" to the production environment for unbuilt upstream resources.

Usage​

dbt run --select [...] --defer --state path/to/artifacts
dbt test --select [...] --defer --state path/to/artifacts

When the --defer flag is provided, dbt will resolve ref calls differently depending on two criteria:

  1. Is the referenced node included in the model selection criteria of the current run?
  2. Does the reference node exist as a database object in the current environment?

If the answer to both is no—a node is not included and it does not exist as a database object in the current environment—references to it will use the other namespace instead, provided by the state manifest.

Ephemeral models are never deferred, since they serve as "passthroughs" for other ref calls.

When using defer, you may be selecting from production datasets, development datasets, or a mix of both. Note that this can yield unexpected results

  • if you apply env-specific limits in dev but not prod, as you may end up selecting more data than you expect
  • when executing tests that depend on multiple parents (e.g. relationships), since you're testing "across" environments

Deferral requires both --defer and --state to be set, either by passing flags explicitly or by setting environment variables (DBT_DEFER and DBT_STATE). If you use dbt Cloud, read about how to set up CI jobs.

Favor state​

You can optionally skip the second criterion by passing the --favor-state flag. If passed, dbt will favor using the node defined in your --state namespace, even if the node exists in the current target.

Example​

In my local development environment, I create all models in my target schema, dev_alice. In production, the same models are created in a schema named prod.

I access the dbt-generated artifacts (namely manifest.json) from a production run, and copy them into a local directory called prod-run-artifacts.

run​

I've been working on model_b:

models/model_b.sql
select

    id,
    count(*)

from {{ ref('model_a') }}
group by 1

I want to test my changes. Nothing exists in my development schema, dev_alice.

test​

dbt run --select "model_b"
target/run/my_project/model_b.sql
create or replace view dev_me.model_b as (

    select

        id,
        count(*)

    from dev_alice.model_a
    group by 1

)

Unless I had previously run model_a into this development environment, dev_alice.model_a will not exist, thereby causing a database error.

I also have a relationships test that establishes referential integrity between model_a and model_b:

models/resources.yml
version: 2

models:
  - name: model_b
    columns:
      - name: id
        tests:
          - relationships:
              to: ref('model_a')
              field: id

(A bit silly, since all the data in model_b had to come from model_a, but suspend your disbelief.)

dbt test --select "model_b"
target/compiled/.../relationships_model_b_id__id__ref_model_a_.sql
select count(*) as validation_errors
from (
    select id as id from dev_alice.model_b
) as child
left join (
    select id as id from dev_alice.model_a
) as parent on parent.id = child.id
where child.id is not null
  and parent.id is null

The relationships test requires both model_a and model_b. Because I did not build model_a in my previous dbt run, dev_alice.model_a does not exist and this test query fails.

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