How to Fix “The preview is being determined” Stuck in Power Query Editor

The message "The preview is being determined..." appears in the Power Query Editor while the mashup engine evaluates incoming records to render the first rows of your table. If this message remains stuck indefinitely, Power Query is locked in an intensive background evaluation loop. This is usually caused by heavy column profiling, broken query folding that forces the engine to download millions of raw records, or background preview downloads overwhelming local resources.

Fast Fix

In the Power Query Editor, go to the View tab and uncheck Column quality, Column distribution, and Column profile. At the bottom of the window in the status bar, ensure the profiling setting is set to Column profiling based on top 1000 rows, not the entire dataset. Finally, disable background data downloads under Query Options > Data Load.

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Diagnostic Snapshot

Property	Details
Symptom	Endless progress bar, “The preview is being determined…”, or spinning evaluation wheel
Affected Area	Main preview grid in the Power Query Editor
Primary Root Cause	Column profiling scanning millions of records, or un-folded queries downloading entire remote tables
System Impact	High CPU/RAM usage by Microsoft.Mashup.Container.* processes

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Why the Preview Process Freezes

When you select a step in the Applied Steps pane, Power Query does not just display static data. It spins up a background worker to evaluate the upstream M-code and generate a sample preview.

[Remote Source: 2M Rows]
            │
            ▼
[Power Query Preview Evaluator]
            │
    Is Profiling Enabled Across Full Dataset?
    ├── YES ──► Downloads all 2M rows locally ──► FREEZES: "Preview being determined"
    └── NO  ──► Reads only first 1,000 rows   ──► RENDERS INSTANTLY

The preview freezes under three common conditions:

1. Full-Dataset Column Profiling: If you turn on Column Quality or Column Value Distribution and set it to evaluate the entire dataset, Power Query cannot render a single cell until it has downloaded, parsed, and calculated metrics for every row in the source.
2. Broken Query Folding on Massive Sources: If your query contains steps that cannot be translated into native SQL (like custom text manipulations or cross-joins), Power Query must download the complete dataset from the server to evaluate a 10-row preview.
3. Background Previews for Multiple Queries: When you open a workbook with 20 queries, Excel attempts to refresh the preview cache for all of them in parallel, exhausting memory and locking the UI.

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Step-by-Step Resolution

Step 1: Disable Intensive Column Profiling

Column profiling tools provide valuable insights into null values and data distributions, but they can bring the editor to a halt on large datasets.

1. In the Power Query Editor, click the View tab in the top ribbon.
2. Look at the Data Preview group.
3. Uncheck all three options:
   • [ ] Column quality
   • [ ] Column distribution
   • [ ] Column profile
4. Look at the bottom-left corner of the window in the Status Bar.
5. If it says “Column profiling based on entire data set”, click the text and change it to Column profiling based on top 1000 rows.

[Power Query View Tab]
  Data Preview Group:
  [ ] Column quality       <-- Uncheck
  [ ] Column distribution   <-- Uncheck
  [ ] Column profile        <-- Uncheck

Status Bar (Bottom):
  Click: "Column profiling based on entire data set" ──► Change to: [Top 1000 rows]

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Step 2: Disable Background Preview Downloads

By default, Power Query downloads extra data in the background to make future navigation feel faster. On large files or slow connections, this causes severe preview lag.

1. In Excel, navigate to Data > Get Data > Query Options.
2. Under Current Workbook, select Data Load.
3. Under the Background Data section, uncheck:
   • [ ] Allow data preview to download in the background
4. Under Global > Data Load, repeat this step:
   • Select Never allow data previews to download in the background.
5. Click OK.

[Query Options]
  └── Current Workbook
        └── Data Load
              └── Background Data
                    └── [ ] Allow data preview to download in the background

Disabling this setting forces Power Query to evaluate only the active query you are currently inspecting.

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Step 3: Insert an Early Row Filter (Temporary Circuit Breaker)

If you are designing a complex query against a slow web API or multi-million-row database, do not let the preview process evaluate the full source while you build your transformation steps:

1. In the Applied Steps pane, click on the Source or Navigation step.
2. Go to the Home tab and click Keep Rows > Keep Top Rows.
3. Enter 100 and click OK.
4. Drag this Kept First Rows step to the very top of your Applied Steps list, immediately below Navigation.
5. Build all your downstream business logic, transforms, and formulas quickly without preview lag.
6. Once the query is finalized, delete the temporary Kept First Rows step before clicking Close & Load.

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Step 4: Check for Broken Query Folding

If you are querying a SQL database, make sure your early steps are folding back to the server:

1. Right-click the step that gets stuck on the preview.
2. If View Native Query is grayed out, that step is forcing Power Query to download all data across the network before rendering the preview.
3. Move simple filters (Date >= 2025-01-01) above any custom column additions or data-type changes.

If broken folding causes queries to time out completely, see Troubleshooting Power Query “Timeout” Errors on Large SQL Datasets. If background refresh conflicts lock the workbook, review Troubleshooting “Background Refresh” Conflicts with Pivot Tables. For memory crashes during preview rendering, see Troubleshooting “Evaluation ran out of memory” in the Power Query Engine. For open file locks, consult How to Fix “The process cannot access the file because it is being used” (PQ).