Over-fetching and under-fetching problems in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What data does this GraphQL query return?

Given the following GraphQL query requesting user details and their posts, what is the exact data returned?

GraphQL

query {
  user(id: "1") {
    id
    name
    posts {
      id
      title
    }
  }
}

A {"data":{"user":{"id":"1","name":"Alice","posts":[{"id":"101","title":"GraphQL Basics"},{"id":"102","title":"Advanced GraphQL"}]}}}

B {"data":{"user":{"id":"1","name":"Alice"}}}

C {"data":{"user":{"name":"Alice","posts":[{"id":"101","title":"GraphQL Basics"}]}}}

D {"data":{"user":{"id":"1","posts":[{"id":"101","title":"GraphQL Basics"}]}}}

💡 Hint

Check which fields are requested inside the user object.

The query requests id, name, and posts with id and title. So the response includes all these fields.

🧠 Conceptual

intermediate

Which problem does this GraphQL query cause?

This GraphQL query requests all fields of a user and all fields of their posts, including large content fields. What problem does this cause?

GraphQL

query {
  user(id: "1") {
    id
    name
    email
    posts {
      id
      title
      content
      comments {
        id
        text
      }
    }
  }
}

A No problem: The query is perfectly optimized.

B Under-fetching: The query misses some fields needed by the client.

C Syntax error: The query is invalid due to missing brackets.

D Over-fetching: The query requests more data than needed, causing slow responses.

💡 Hint

Think about the size and amount of data requested.

Requesting all fields including large content and nested comments can cause over-fetching, slowing down the response and wasting bandwidth.

📝 Syntax

advanced

Identify the syntax error in this GraphQL query

Find the syntax error in the following GraphQL query that fetches user and posts data.

GraphQL

query {
  user(id: "1") {
    id
    name
    posts {
      id
      title
    }
  
}

A Missing closing brace for the user field.

B Missing comma between fields.

C Using parentheses instead of braces for posts.

D Missing quotes around the id value.

💡 Hint

Count the opening and closing braces carefully.

The query is missing the closing brace for the user field, causing a syntax error.

❓ optimization

advanced

How to fix under-fetching in this GraphQL query?

The client needs the user's email but the query only requests id and name. How to fix this under-fetching problem?

GraphQL

query {
  user(id: "1") {
    id
    name
  }
}

A Remove the id field from the query.

B Add the email field inside the user selection set.

C Add a separate query for email outside user.

D Use a mutation instead of a query.

💡 Hint

Think about what fields the client needs to see.

Under-fetching happens when the query misses fields the client needs. Adding email fixes this.

🧠 Conceptual

expert

Why does GraphQL help solve over-fetching and under-fetching?

Choose the best explanation for how GraphQL addresses over-fetching and under-fetching problems compared to REST APIs.

A GraphQL automatically caches all data to prevent over-fetching.

B GraphQL forces clients to fetch all data in one request, preventing under-fetching.

C GraphQL lets clients specify exactly which fields they want, avoiding extra or missing data.

D GraphQL uses fixed endpoints that return fixed data sets.

💡 Hint

Think about how GraphQL queries differ from REST endpoints.

GraphQL allows clients to request only the data they need, solving both over-fetching and under-fetching issues common in REST.