How do you detect the N+1 query problem?

N+1 = 1 query for the parent + 1 per child row when accessing a navigation property.

Detection methods, in order of investment:

1. EF Core SQL logging (dev):

options.LogTo(Console.WriteLine, LogLevel.Information);

Look for the same SELECT shape repeated dozens of times in rapid succession. Each one differs only by the WHERE clause value.

2. Application Performance Monitoring (production): Datadog, Honeycomb, Application Insights, OpenTelemetry — they show a "sawtooth" of identical small DB spans on a single request. One trace tells you immediately.

3. Test-level query counting:

// Wrap commands with a counter; assert max queries per request.
var queryCount = 0;
db.SavingChanges += (_, _) => queryCount++;

var orders = await service.ListOrdersAsync();
Assert.That(queryCount, Is.LessThan(3));

Catches regressions in CI.

4. EXPLAIN ANALYZE the slow query: Sometimes "N+1" is actually a single slow query missing an index. Run EXPLAIN ANALYZE to see the query plan.

Why it bites in production but not dev:

• Localhost network latency is microseconds → 100 queries feel instant
• Production network is milliseconds × 100 = whole seconds added
• Dev data is small → small Cartesian products

The fix — always one of:

• .Include() for navigation properties you'll access
• .AsSplitQuery() for multi-collection includes
• .Select(o => new Dto { ... }) projection — usually best

Rule: if any production request spawns more than 5 SQL queries, treat it as a defect until proven otherwise.