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# 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.
A DB 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
SELECT "project_features".* FROM "project_features" WHERE "project_features"."project_id" = $1 LIMIT $2 [["project_id", 5], ["LIMIT", 1]] /*application:web,controller:jobs,action:trace,correlation_id:rYF4mey9CH3,line:/app/policies/project_policy.rb:504:in `feature_available?'*/
```
1. Sidekiq:
```sql
SELECT "ci_pipelines".* FROM "ci_pipelines" WHERE "ci_pipelines"."id" = $1 LIMIT $2 [["id", 64], ["LIMIT", 1]] /*application:sidekiq,jid:e7d6668a39a991e323009833,job_class:ExpireJobCacheWorker,correlation_id:rYF4mey9CH3,line:/app/workers/expire_job_cache_worker.rb:14:in `perform'*/
```
## Enable/Disable the feature
Enabling or disabling the feature requires a **restart/SIGHUP** of the Web and
Sidekiq workers, as the feature flag's state is memoized upon starting up.
The `feature_flag` for this feature is **disabled** by default. You can enable
or disable it with:
```ruby
Feature.enable(:marginalia)
Feature.disable(:marginalia)
```
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