# Core Execution Actions

## Preview vs. Run vs. Test vs. Build

When developing and maintaining data pipelines in dex, you'll frequently interact with four core actions: **Preview**, **Run**, **Test**, and **Build**. Each serves a distinct purpose in the model development lifecycle and helps you validate, execute, or deploy your transformations with clarity and control.

Below is a breakdown of what each action does and when to use it:

### Preview

**Preview** lets you see the result of a model’s `SELECT` statement without materializing the output to your warehouse.

Use this to:

* Quickly inspect a model’s logic
* Validate column values and row-level transformations
* Debug joins, filters, and aggregations

{% hint style="info" %}
Preview does **not** create or update a table or view. It's lightweight, fast, and ideal during active development.
{% endhint %}

### Run

**Run** executes the model and materializes it based on its configured materialization (`view`, `table`, `incremental`, or `ephemeral`).

Use this when:

* You want to persist the model’s result in your data warehouse
* You're ready to push updates to downstream models
* You need to trigger the full SQL or Python logic in a production-like setting

Run respects model dependencies defined using `ref()` and will execute upstream models as needed.

### Test

**Test** runs one or more data quality assertions defined in your project.

Tests are used to:

* Validate that your data meets expected conditions
* Catch issues like nulls, duplicates, invalid relationships, or custom logic mismatches
* Monitor the health of your pipeline

You can test a single model or run all tests in a project. Failing tests will be logged and surfaced in the UI in the Develop and Flows tab.

### Build

**Build** is a comprehensive action that:

1. Runs the model (materializes it)
2. Runs all associated tests
3. Respects full model dependencies

Use Build when:

* You're ready to promote or validate a complete transformation layer (e.g., a `3.gold` model)
* You want a one-click way to execute and verify your logic end to end
* You're preparing models for deployment in automated Flows or production environments

### Summary Table

| Action  | Persists Data | Runs SQL | Executes Tests |
| ------- | ------------- | -------- | -------------- |
| Preview | ❌             | ✅        | ❌              |
| Run     | ✅             | ✅        | ❌              |
| Test    | ❌             | ❌        | ✅              |
| Build   | ✅             | ✅        | ✅              |

### Understanding Upstream and Downstream

dex models are connected through **dependencies**, which define the order of execution in your Flows.

* **Upstream** models are the ones your current model depends on. If model `B` references model `A` using `{{ ref('A') }}`, then `A` is upstream of `B`.
* **Downstream** models depend on the current model. Continuing the example, `B` would be downstream of `A`.

These relationships impact:

* **Run order**: Upstream models are always built first.
* **Lineage**: dex visualizes model dependencies to help you trace impact and troubleshoot issues.
* **Build scope**: When you build a model, all its upstream dependencies will also be built automatically.

Understanding upstream and downstream relationships is critical when:

* Debugging broken models
* Making schema changes
* Deciding where to place tests
* Organizing your model layers


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