Field-level cost analysis in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Calculate total cost for selected fields

Given a GraphQL query requesting name and price fields of products, and a cost model where name costs 1 unit and price costs 3 units per item, what is the total cost for 5 products?

GraphQL

query {
  products {
    name
    price
  }
}

A 20

B 15

C 10

D 25

💡 Hint

Multiply the cost per field by the number of products and sum.

Each product requests two fields: name (1 unit) and price (3 units). Total per product is 4 units. For 5 products, total cost is 4 * 5 = 20 units.

🧠 Conceptual

intermediate

Understanding cost impact of nested fields

In a GraphQL query, a user has a nested posts field. If user fields cost 2 units each and posts fields cost 1 unit each, which factor most increases total cost?

A Increasing number of users queried

B Increasing number of <code>user</code> fields requested

C Increasing number of posts per user

D Increasing number of <code>posts</code> fields requested

💡 Hint

Think about how nested lists multiply cost.

More posts per user multiply the cost of posts fields by the number of posts, increasing total cost significantly.

📝 Syntax

advanced

Identify syntax error in cost directive usage

Which option contains a syntax error in applying a cost directive to a GraphQL field?

GraphQL

type Query {
  products: [Product] @cost(multipliers: ["first"])
}

type Product {
  name: String @cost(value: 1)
  price: Float @cost(value: 2)
}

A Using @cost(value=1) on <code>price</code> field

B Using @cost(multipliers: ["first"]) on <code>products</code> field

C Using @cost(value: 1) on <code>name</code> field

D Using @cost(value: 2) on <code>price</code> field

💡 Hint

Check the syntax for directive arguments in GraphQL.

GraphQL directive arguments use colon :, not equals =. Option A uses =, causing syntax error.

❓ optimization

advanced

Optimize query cost by reducing field requests

You have a query requesting id, name, description, and price fields for 10 products. Costs per field are: id = 1, name = 2, description = 5, price = 3. Which option reduces total cost the most while keeping id and price?

A Remove <code>name</code> only

B Remove <code>name</code> and <code>description</code>

C Remove <code>price</code> only

D Remove <code>description</code> only

💡 Hint

Calculate cost savings for each removal.

Removing name and description saves (2 + 5) * 10 = 70 units, more than removing name only (20 units) or description only (50 units). Removing price is not allowed.

🔧 Debug

expert

Diagnose unexpected high cost in nested query

A query requests users with nested posts and comments. Each user has 3 posts, each post has 4 comments. Costs: user fields = 2 units each, post fields = 1 unit each, comment fields = 0.5 units each. The query requests 2 fields per user, 3 fields per post, and 2 fields per comment. Why is the total cost unexpectedly high?

A The query requests too many user fields

B The number of users queried is higher than expected

C The cost per comment field is set too low

D The cost calculation multiplies fields by number of comments incorrectly

💡 Hint

Consider how nested lists multiply total cost.

The total cost multiplies the number of fields by the count of nested items. With 3 posts per user and 4 comments per post, the comment fields multiply by 3 * 4 = 12 per user, increasing cost significantly.

Practice

1. What is the main purpose of using the @cost directive in GraphQL field-level cost analysis?

easy

A. To rename a field in the schema

B. To define the data type of a field

C. To specify the default value of a field

D. To assign a numeric cost to each field to track resource usage

Solution

1. Step 1: Understand the purpose of field-level cost analysis

   Field-level cost analysis helps monitor and limit resource use by assigning costs to fields.

2. Step 2: Identify the role of the @cost directive

   The @cost directive assigns a numeric complexity cost to each field to estimate query cost.

3. Final Answer:

   To assign a numeric cost to each field to track resource usage -> Option D

4. Quick Check:

   @cost assigns cost = A [OK]

Hint: Remember: @cost tracks resource use per field [OK]

Common Mistakes:

• Confusing cost with data type definition
• Thinking @cost renames fields
• Assuming it sets default values

2. Which of the following is the correct syntax to add a cost directive with complexity 5 to a GraphQL field named books?

easy

A. books: [Book] @cost(complexity: 5)

B. books: [Book] @cost(5)

C. books: [Book] @cost(complexity=5)

D. books: [Book] @cost { complexity: 5 }

Solution

1. Step 1: Recall the correct directive syntax

   The @cost directive uses parentheses with named arguments, e.g., @cost(complexity: 5).

2. Step 2: Check each option's syntax

   books: [Book] @cost(complexity: 5) uses correct syntax with named argument and colon. The other three options use incorrect syntax forms.

3. Final Answer:

   books: [Book] @cost(complexity: 5) -> Option A

4. Quick Check:

   Correct directive syntax = B [OK]

Hint: Use parentheses and colon for directive args: @cost(complexity: 5) [OK]

Common Mistakes:

• Using equal sign instead of colon
• Omitting parentheses
• Using braces instead of parentheses

3. Given the schema snippet:

type Query {
  users: [User] @cost(complexity: 2, multipliers: ["first"])
}

input UserFilter {
  first: Int
}

And the query:

{ users(first: 3) { id name } }

What is the total cost of this query assuming id and name fields have cost 1 each?

medium

A. 3

B. 6

C. 8

D. 5

Solution

1. Step 1: Calculate base complexity and multipliers

   Base complexity is 2. The multiplier is the argument "first" with value 3, so multiply 2 * 3 = 6.

2. Step 2: Add cost of requested fields

   Fields id and name each cost 1, total 2. Add to 6 gives 8.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   2 * 3 + 1 + 1 = 8 [OK]

Hint: Multiply complexity by argument, then add field costs [OK]

Common Mistakes:

• Ignoring multipliers
• Not adding field costs
• Multiplying fields cost instead of adding

4. Consider this incorrect directive usage:

type Query {
  posts: [Post] @cost(complexity: "high")
}

What is the main error here?

medium

A. The complexity value must be an integer, not a string

B. The field name posts is invalid

C. The directive @cost cannot be used on lists

D. The directive syntax is missing parentheses

Solution

1. Step 1: Check the type of complexity argument

   The complexity argument expects an integer value, but "high" is a string.

2. Step 2: Verify other parts of the directive usage

   The field name and directive usage are valid; parentheses are present.

3. Final Answer:

   The complexity value must be an integer, not a string -> Option A

4. Quick Check:

   Complexity expects integer = D [OK]

Hint: Complexity must be a number, not text [OK]

Common Mistakes:

• Using string instead of integer for complexity
• Thinking directive can't be on lists
• Missing parentheses in directive

5. You have a GraphQL field comments with @cost(complexity: 1, multipliers: ["limit"]). The query requests comments(limit: 4) with subfields text and author, each costing 2. What is the total cost of this query?

hard

A. 12

B. 8

C. 10

D. 6

Solution

1. Step 1: Calculate base complexity with multiplier

   Base complexity is 1. Multiplier is argument "limit" with value 4, so 1 * 4 = 4.

2. Step 2: Add cost of subfields

   Subfields text and author each cost 2, total 4. Add to 4 gives 8.

3. Step 3: Verify the total

   Subfields are added flatly without further multiplication by list size: 4 (base) + 4 (subfields) = 8.

4. Final Answer:

   8 -> Option B

5. Quick Check:

   1*4 + (2+2) = 8 [OK]

Hint: Add base cost times multiplier plus subfields cost [OK]

Common Mistakes:

• Multiplying subfields cost by multiplier (e.g., getting 20)
• Adding costs without multiplier
• Confusing which costs to multiply