---
stage: Enablement
group: Database
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
---

# Database query comments with Marginalia

The [Marginalia gem](https://github.com/basecamp/marginalia) is used to add
query comments containing application related context information to PostgreSQL
queries generated by ActiveRecord.

It is very useful for tracing problematic queries back to the application source.

An engineer during an on-call incident will have the full context of a query
and its application source from the comments.

## Metadata information in comments

Queries generated from **Rails** include the following metadata in comments:

- `application`
- `controller`
- `action`
- `correlation_id`
- `line`

Queries generated from **Sidekiq** workers will include the following metadata
in comments:

- `application`
- `jid`
- `job_class`
- `correlation_id`
- `line`

Examples of queries with comments as observed in `development.log`:

1. Rails:

   ```sql
   /*application:web,controller:jobs,action:trace,correlation_id:rYF4mey9CH3,line:/app/policies/project_policy.rb:504:in `feature_available?'*/ SELECT "project_features".* FROM "project_features" WHERE "project_features"."project_id" = $1 LIMIT $2 [["project_id", 5], ["LIMIT", 1]]
   ```

1. Sidekiq:

   ```sql
   /*application:sidekiq,jid:e7d6668a39a991e323009833,job_class:ExpireJobCacheWorker,correlation_id:rYF4mey9CH3,line:/app/workers/expire_job_cache_worker.rb:14:in `perform'*/ SELECT "ci_pipelines".* FROM "ci_pipelines" WHERE "ci_pipelines"."id" = $1 LIMIT $2 [["id", 64], ["LIMIT", 1]]
   ```