N+1 query problem

The N+1 query problem occurs when one query returns N records and application code performs another query for each record.

sequenceDiagram
    participant App as Application
    participant DB as Database
    App->>DB: Load N parent records
    DB-->>App: N parent records
    loop Once per parent
        App->>DB: Load related data
        DB-->>App: Related data
    end

A common application shape looks like this:

n-plus-one.ts

const games = await repository.loadGames();

for (const game of games)
{
    game.offers = await repository.loadOffers(game.id);
}

The loop turns one initial query into one additional round trip per game.

Symptoms

Latency and database load grow with result size. A page that looks correct in small tests can become slow under realistic data volume.

Detection

Count database round trips for one request. Use query logs, tracing, or ORM diagnostics to identify repeated statements that differ only by one key.

Measure round trips, not only query time

Each individual query can be fast while the request is slow because the application performs too many sequential database round trips.

Mitigation

Load related data in a bounded set of queries. Suitable techniques include joins, explicit eager loading, batch loaders, prefetching, or a query shaped for the read model.

Trade-offs

Aggressive joins can duplicate rows, increase memory use, or fetch data that callers do not need. Eager loading every relation can replace many small queries with one excessively large query.

Optimize the access pattern that the caller actually uses.

Related knowledge

Bounded workPlace explicit limits on batch size, concurrency, queue depth, and scan range so one operation cannot consume unbounded resources.

Latency vs throughputDistinguish how long one operation takes from how much work a system completes over time, because optimizing one can worsen the other.

CachingReuse a previously computed or fetched result to avoid repeating expensive work, at the cost of managing staleness and invalidation.