# Binary Operators and Vector Matching — Prometheus + Grafana

Source: https://www.skillbyai.com/en/prometheus-grafana/a-match

> Combining two sets of series.

## on, ignoring and group_left

Arithmetic and comparison operators between two instant vectors match series with **identical label sets**. Use `on(labels)` to match only on some labels or `ignoring(labels)` to ignore some; for many-to-one matches add `group_left` (or `group_right`) and optionally copy labels from the "one" side. A common pattern joins an **info metric** (value 1, with descriptive labels) to add labels such as a version to other series. Comparison operators filter series unless `bool` is added.

## Matching examples

PromQL.

```promql
# memory used as a fraction of the container limit (both share pod and container labels)
sum by (pod, container) (container_memory_working_set_bytes)
  / on (pod, container)
sum by (pod, container) (kube_pod_container_resource_limits{resource="memory"})

# add the app version from an info metric to request rates
sum by (instance) (rate(http_requests_total[5m]))
  * on (instance) group_left (version)
app_build_info

# keep only hosts above 90% disk usage
(1 - node_filesystem_avail_bytes / node_filesystem_size_bytes) > 0.9
```

## Check labels on both sides first

Run each side separately and compare label sets; most "no data" results are matching problems.

**Quiz:** What does group_left enable?

- [ ] Removing all labels
- [ ] Sorting results
- [ ] Joining logs
- [x] Many-to-one matching, where several left series match one right series

*Answer:* Many-to-one matching, where several left series match one right series. Needed for info-metric joins.