Performance optimization: eliminate N+1 queries, add indexes, implement database-level pagination

[Claude]

Identified and fixed critical performance bottlenecks causing excessive database queries, full table scans, and in-memory processing of large datasets.

Changes

N+1 Query Elimination

• Controllers: Refactored GetCourseRegistrations and GetStudentRegistrations to eliminate redundant existence checks
  • Before: 2 queries (verify entity exists → fetch registrations)
  • After: 1 query (fetch registrations → lazy validation only if empty)

Database-Level Pagination

• Services: CourseService.GetCoursesAsync and RegistrationService.GetRegistrationsAsync now paginate at database level
• Repositories: Added SearchCoursesPagedAsync and GetRegistrationsWithFiltersPagedAsync methods
  • Before: SELECT * → Count() in memory → Skip/Take in memory
  • After: SELECT COUNT(*) → SELECT * OFFSET/FETCH at database
  • Impact: For 100k records with page size 10, loads 10 rows instead of 100k (99.99% reduction)

// Before: loads all matching courses into memory
courses = await SearchCoursesAsync(searchTerm, instructor);
totalCourses = courses.Count();  // enumerated in memory
courses = courses.Skip((page - 1) * pageSize).Take(pageSize);

// After: pagination at database level
var result = await SearchCoursesPagedAsync(searchTerm, instructor, page, pageSize);
courses = result.Courses;  // only requested page loaded
totalCourses = result.TotalCount;  // from COUNT(*) query

Database Indexes

Added 7 performance indexes in CourseRegistrationDbContext:

• Students.IsActive, Courses.IsActive, Courses.InstructorName
• Courses.IsActive + StartDate (composite for available courses query)
• Registrations.Status, Registrations.StudentId, Registrations.CourseId

Query Optimization

• AsNoTracking: Applied to all read-only queries in CourseRepository (5 methods)
• Selective loading: Removed unnecessary .Include(c => c.Registrations) from GetAvailableCoursesAsync that was loading 100k+ registration records

Migration Required

cd api/CourseRegistration.Infrastructure
dotnet ef migrations add AddPerformanceIndexes --startup-project ../CourseRegistration.API
dotnet ef database update --startup-project ../CourseRegistration.API

Documentation

• PERFORMANCE_IMPROVEMENTS.md - detailed analysis with benchmarks
• PERFORMANCE_SUMMARY.md - quick reference guide

Impact Estimates

• Paginated searches: 95%+ reduction in data transfer and memory
• Indexed queries: 90%+ faster execution (index seeks vs table scans)
• N+1 elimination: 50% fewer database round-trips
• AsNoTracking: ~30% memory reduction per query