# Overview

Metric aggregations compute summary numbers over all documents that matched your `filter`.

Use metrics when you want one value (or stats object), not grouped buckets.

### Available Operators

| Operator | What it computes |
|----------|------------------|
| [`$avg`](./avg) | Arithmetic mean |
| [`$sum`](./sum) | Total of values |
| [`$min`](./min) | Smallest value |
| [`$max`](./max) | Largest value |
| [`$count`](./count) | Count of values |
| [`$cardinality`](./cardinality) | Count of distinct values |
| [`$stats`](./stats) | `count`, `sum`, `min`, `max`, `avg` |
| [`$extendedStats`](./extended-stats) | `$stats` + variance and std deviation metrics |
| [`$percentiles`](./percentiles) | Distribution percent points |

### Input Format

Every metric operator takes an object with at least a `field` property:

```json
{"alias_name": {"$avg": {"field": "price"}}}
{"alias_name": {"$avg": {"field": "price", "missing": 0}}}
```

The `field` value must be a string pointing to a FAST field in your schema.

### Behavior Notes

- Metric operators require a `field`.
- The field must be `FAST` in your schema.
- Metric operators do not support nested `$aggs`.
- For many metric operators, `missing` lets you provide a fallback value for documents where the field does not exist.