[FAQ Bot] NEW: Why do Grafana pie charts show n or only one feedback slice even when

_questions/llm-zoomcamp/module-5/008_0cf2421dca_grafana-pie-charts-rows-to-fields-relevance-feedba.md

@@ -0,0 +1,40 @@
+---
+id: 0cf2421dca
+question: Why do Grafana pie charts show `n` or only one feedback slice even when
+  the PostgreSQL query returns separate relevance/feedback categories?
+sort_order: 8
+---
+
+If your PostgreSQL query returns multiple categories as multiple rows (e.g., `feedback`, `n`), but your pie chart is configured/reducing to a single numeric field (often `n`), Grafana may collapse the rows and end up with only one slice.
+
+Fix: use the `Rows to fields` transformation so each category becomes its own numeric field.
+
+In the panel editor:
+1) Set the PostgreSQL query `Format` to `Table` and ensure it returns one row per category, with:
+   - a text category column (e.g., `feedback`)
+   - a numeric count column (e.g., `n`)
+2) Open `Transformations` → `Add transformation` → `Rows to fields`.
+3) In the field mapping table:
+   - set the category column (e.g., `feedback`) to `Use as` → `Field name`
+   - set the count column (e.g., `n`) to `Use as` → `Field value`
+4) Use the resulting fields in the pie chart.
+   - With one count per field, use `Calculate` → `Last (not null)` to keep each category as its own slice.
+   - Enable legend names/values to verify you get e.g. `thumbs up: 3` and `thumbs down: 2`.
+
+Example read-only query (two feedback categories):
+```SQL
+WITH feedback_counts(feedback, n) AS (
+VALUES ('thumbs up', 3), ('thumbs down', 2)
+)
+SELECT feedback, n
+FROM feedback_counts;
+```
+
+Why this happens: Grafana can reduce a numeric column named `n` to a single value (often the last one), which produces one slice. Renaming labels alone doesn’t split the data into separate numeric fields per category—`Rows to fields` does.
+
+Note: this kind of transform doesn't populate an empty result; validate using real aggregates from the same DB and time range you're charting. The example above should display `thumbs up: 3` and `thumbs down: 2` (5 votes total), not one slice labelled `n`. This was verified with Grafana 10.2.3 and a PostgreSQL datasource.
+
+References:
+
+- [Grafana: Rows to fields transformation](https://grafana.com/docs/grafana/latest/panels-visualizations/query-transform-data/transform-data/#rows-to-fields)
+- [Grafana 10.2.3 rowsToFields implementation](https://github.com/grafana/grafana/blob/v10.2.3/public/app/features/transformers/rowsToFields/rowsToFields.ts)