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# Preview a pipeline task before you apply it

> See a live preview of a Mammoth pipeline task before you apply it: result rows, changed values, removed rows, duplicates and join matches.
>
> Applies to: Where: View › Describe your transformation › (function) › Preview

Most functions show a live preview of their result in a window above the configuration panel, before you click **Apply**. The preview never changes your data. It opens once the settings are complete; until then it reads "Complete the settings to see a preview." and it shows "Updating…" while you change them.



## Why the preview uses a sample of rows

To answer quickly, the preview reads a sample of the View. The line under it reads "Uses the first" with the sample size and the View's row count, or "Uses all" with the row count when it read every row. With a sample, counts in the preview describe the sample, not the whole View, and the summary says so.

## Turn the preview on or off

| Part of the panel header | What it does |
| --- | --- |
| **Preview** (eye icon) | Turns the preview off or on. Each task opens with it on; turning it off holds for that task only. |
| Summary beside it | Says what the step changes, for example "120 rows removed", "2 columns added", "No changes" or "500 → 12 rows". It turns into a warning when every row would be removed or the settings have issues. Click it to minimize or reopen the window. |

You can move, resize, minimize, expand and close the window. Double-click the title bar to expand it. Keyboard users move or resize it with the arrow keys; **Home** restores its default place and size.

## What each preview tab shows

**Result** is always there. The other tabs appear when they apply to the function.

| Tab | What it shows |
| --- | --- |
| **Result** | The first rows of the result, with new columns marked. |
| **Changes** | For a step that changes values: how many rows change, with before and after values. Shows as, for example, **Changes: revenue** when several columns change. |
| **Values** | For a step that adds a column: the values it creates and how many rows get each one. |
| **Removed rows** | The rows a filter or limit removes. |
| **Duplicates** | The duplicate rows Remove Duplicate Rows finds. The last row of each group stays. |
| **Groups** | For a step that works on groups of rows separately: how many groups there are, and the rows of the first ones. |
| **Matches** | For Join and Lookup: rows that match, rows with no match, and keys with no match. It also says when a row matches more than one row of the other View (Join adds a row for each match) or a key of the other View has more than one value (Lookup takes the first). |
| **Size** | For Repeat Rows: shown only when the result is large, too large to apply, or empty. |
| **Views** | For Join and Lookup: this View and the other View side by side. |

## Functions that preview on request or not at all

| Function | Preview |
| --- | --- |
| **Fill Missing Values** with AI fill | Waits for you: "This function takes time to run. Click Run preview to see its result." Click **Run preview**. |
| **Generative AI** | Its own **Preview** button. |
| **Send to Dataset**, **Bulk Replace**, the Export step, **Data Check**, **Checkpoint Alert** | No live preview. |

## Troubleshooting the task preview

| Message | What to do |
| --- | --- |
| "The preview did not finish in time. Try again, or use narrower settings." | Click **Try again**, or narrow the settings (fewer columns, a tighter condition). |
| "The preview could not load." | Click **Try again**. You can still apply the task. |
| "A preview is not available for this function." | Apply the task and check the result in the grid. |
| "No rows in the result." | The step removes every row. Check the settings before applying. |

For a task that fails after you apply it, see [Fix a failing step](https://mammoth.io/docs/learn/clean-and-shape/fix-a-failing-step/).

## Related

- [How a Mammoth pipeline works](https://mammoth.io/docs/learn/clean-and-shape/how-the-pipeline-works/): open the Transform menu and add a task.
- [Join two datasets on a shared column](https://mammoth.io/docs/learn/clean-and-shape/join/): what the Matches tab checks.
- [Column types, IDs, currency and percentages](https://mammoth.io/docs/learn/clean-and-shape/column-types/): the warnings some tasks show about column meaning.

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Source: https://mammoth.io/docs/learn/clean-and-shape/preview-a-task.md · Updated: 2026-11-08