fix(#1013): Eliminate N+1 count queries in KpiService - #1107
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Well done on the job done so far! |
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📝 Description
This pull request resolves Issue #1013 by eliminating severe N+1 query bottlenecks within the KpiService, which previously executed sequential database queries based on the number of courses and cohorts.
🛠️ Changes Made
Enrollment Conversion Optimization: Replaced the per-course enrollmentRepository.count() loop with a single grouped query (SELECT courseId, COUNT(*) FROM enrollment GROUP BY courseId). The results are loaded into memory and mapped to the course list, reducing thousands of sequential queries to exactly two queries.
User Retention Optimization: Rewrote calculateUserRetention to use a direct SQL COUNT aggregation rather than eagerly loading all user rows per cohort month via userRepository.find().
Performance Observability: Enclosed both KPI job executions with a duration tracker, exporting a kpi_job_duration_ms metric using MetricsService. This ensures any future regressions in query performance or job duration are highly visible.
Tests Added: Added kpi.service.spec.ts to assert that the calculateEnrollmentConversionRate issues a constant number of queries (strictly bounded to exactly one aggregate query builder call) regardless of the number of courses dynamically returned.
🧪 Acceptance Criteria Addressed
The conversion rate job issues a constant number of queries regardless of course count.
Retention calculation uses aggregation and does not load individual user rows.
Job duration is exported as a metric for monitoring.
Tests added to assert constant query count.
🔗 Related Issues
Resolves #1013