Query variables in GraphQL - Time & Space Complexity

Time Complexity: Query variables

O(p + p x c)

Understanding Time Complexity

We want to understand how the time to run a GraphQL query changes when we use variables.

Specifically, how does the query's work grow as the input data or variables change?

Scenario Under Consideration

Analyze the time complexity of the following GraphQL query using variables.

query GetUser($id: ID!) {
  user(id: $id) {
    name
    posts {
      title
      comments {
        text
      }
    }
  }
}
    

This query fetches a user by ID, then gets their posts and comments on those posts.

Identify Repeating Operations

Look for repeated actions in the query.

• Primary operation: Fetching posts and then comments for each post.
• How many times: For each post, all comments are fetched, so this repeats for every post.

How Execution Grows With Input

As the number of posts and comments grows, the work grows too.

Input Size (n)	Approx. Operations
10 posts, 5 comments each	About 10 + (10 x 5) = 60 operations
100 posts, 5 comments each	About 100 + (100 x 5) = 600 operations
1000 posts, 5 comments each	About 1000 + (1000 x 5) = 6000 operations

Pattern observation: The total work grows roughly in proportion to the number of posts times the number of comments per post.

Final Time Complexity

Time Complexity: O(p + p x c)

This means the time grows with the number of posts (p) plus the number of comments (c) per post multiplied by posts.

Common Mistake

[X] Wrong: "Using variables makes the query always run in constant time."

[OK] Correct: Variables only change input values; the query still processes all posts and comments, so time depends on data size.

Interview Connect

Understanding how query variables affect time helps you explain performance clearly and shows you know how data size impacts work.

Self-Check

What if we added a variable to limit the number of posts returned? How would that change the time complexity?

Practice

1. What is the main purpose of using query variables in GraphQL?

easy

A. To create new types in the GraphQL schema

B. To define the schema of the GraphQL server

C. To store query results on the client side

D. To reuse the same query with different input values

Solution

1. Step 1: Understand what query variables do

   Query variables allow you to write a query once and provide different inputs each time you run it.

2. Step 2: Compare options with this purpose

   Only To reuse the same query with different input values describes reusing queries with different inputs, which matches the purpose of query variables.

3. Final Answer:

   To reuse the same query with different input values -> Option D

4. Quick Check:

   Query variables = reuse queries with different inputs [OK]

Hint: Variables let you change inputs without rewriting queries [OK]

Common Mistakes:

• Confusing variables with schema definitions
• Thinking variables store data permanently
• Mixing variables with client-side caching

2. Which of the following is the correct way to declare a variable in a GraphQL query?

easy

A. query getUser($id: ID!) { user(id: $id) { name } }

B. query getUser(id: $ID) { user(id: id) { name } }

C. query getUser($id) { user(id: id) { name } }

D. query getUser { user(id: $id) { name } }

Solution

1. Step 1: Identify correct variable syntax

   Variables must be declared with a $ sign and a type, e.g., $id: ID! inside parentheses after the query name.

2. Step 2: Check each option

   query getUser($id: ID!) { user(id: $id) { name } } correctly declares $id: ID! and uses $id inside the query. Others miss type, $ sign, or parentheses.

3. Final Answer:

   query getUser($id: ID!) { user(id: $id) { name } } -> Option A

4. Quick Check:

   Variable declaration = $name: Type! in parentheses [OK]

Hint: Variables need $ and type declared after query name [OK]

Common Mistakes:

• Omitting the variable type
• Using variable without $ inside query
• Not placing variables in parentheses after query name

3. Given the query:

query getBook($bookId: ID!) { book(id: $bookId) { title author } }

and variables:

{ "bookId": "123" }

What will the query return if the book with ID 123 has title "GraphQL Guide" and author "Anna"?

medium

A. { "data": { "book": null } }

B. { "data": { "book": { "title": "GraphQL Guide", "author": "Anna" } } }

C. SyntaxError: Variable not defined

D. { "errors": [ { "message": "Book not found" } ] }

Solution

1. Step 1: Understand the query and variables

   The query asks for a book by ID using variable $bookId. The variable value is "123".

2. Step 2: Match variable value to data

   The book with ID "123" exists with title "GraphQL Guide" and author "Anna". So the query returns this data.

3. Final Answer:

   { "data": { "book": { "title": "GraphQL Guide", "author": "Anna" } } } -> Option B

4. Quick Check:

   Variable value matches book data = correct result [OK]

Hint: Variables fill query inputs to get matching data [OK]

Common Mistakes:

• Assuming variable names are optional
• Expecting errors when data exists
• Confusing null with errors

4. Identify the error in this GraphQL query using variables:

query getUser($userId: ID) { user(id: userId) { name } }

medium

A. Variable userId inside query should have a $ sign

B. Variable $userId is missing the non-null type indicator

C. Variables must be declared outside the query

D. Query name cannot have variables declared

Solution

1. Step 1: Check variable usage inside query

   The variable is declared as $userId but inside the query it is used as userId without the $ sign.

2. Step 2: Correct usage of variables

   Variables must be referenced with the $ sign inside the query fields, so it should be id: $userId.

3. Final Answer:

   Variable userId inside query should have a $ sign -> Option A

4. Quick Check:

   Use $ before variable inside query fields [OK]

Hint: Always use $ before variable inside query body [OK]

Common Mistakes:

• Forgetting $ before variable in query
• Thinking variables declared outside query
• Confusing variable declaration syntax

5. You want to write a GraphQL query to fetch a list of books filtered by optional author name using variables. Which query correctly declares the variable and uses it to filter books only if the variable is provided?

hard

A. query getBooks($author: String!) { books(author: $author) { title } }

B. query getBooks { books(author: $author) { title } }

C. query getBooks($author: String) { books(author: $author) { title } }

D. query getBooks($author) { books(author: author) { title } }

Solution

1. Step 1: Understand optional variable declaration

   To make a variable optional, declare it without the exclamation mark, e.g., $author: String.

2. Step 2: Check variable usage in query

   Use $author inside the query to filter books. query getBooks($author: String) { books(author: $author) { title } } correctly declares and uses the optional variable.

3. Step 3: Review other options

   query getBooks($author: String!) { books(author: $author) { title } } makes variable required with !, C uses variable without declaration, D misses type and uses variable incorrectly.

4. Final Answer:

   query getBooks($author: String) { books(author: $author) { title } } -> Option C

5. Quick Check:

   Optional variable = no ! and use $ in query [OK]

Hint: Optional variables omit ! and use $ inside query [OK]

Common Mistakes:

• Making optional variables required with !
• Using variables without declaration
• Forgetting $ in query usage