Persisted queries in GraphQL - Time & Space Complexity

Time Complexity: Persisted queries

O(n)

Understanding Time Complexity

When using persisted queries in GraphQL, we want to understand how the time to find and run a saved query changes as we store more queries.

We ask: How does the system's work grow when the number of saved queries grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

query GetUser($id: ID!) {
  user(id: $id) {
    id
    name
  }
}

# Persisted queries are stored with a unique key.
# When a request comes, the server looks up the query by key and executes it.
# This example shows the lookup step.
    

This snippet represents a typical persisted query lookup by a unique key before execution.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Searching for the query by its key in the stored collection.
• How many times: Once per request, but the search may scan multiple stored queries depending on data structure.

How Execution Grows With Input

As the number of saved queries grows, the time to find one depends on how the queries are stored.

Input Size (n)	Approx. Operations
10	About 10 checks if stored in a list
100	About 100 checks if stored in a list
1000	About 1000 checks if stored in a list

Pattern observation: If stored in a simple list, the search grows linearly with the number of saved queries.

Final Time Complexity

Time Complexity: O(n)

This means the time to find a persisted query grows directly with how many queries are saved if using a simple search.

Common Mistake

[X] Wrong: "Looking up a persisted query always takes the same time no matter how many queries are saved."

[OK] Correct: If the queries are stored in a list or array, the server may need to check many entries, so time grows with the number of queries.

Interview Connect

Understanding how data lookup time grows helps you design systems that stay fast as they grow. This skill is useful when working with APIs and databases in real projects.

Self-Check

What if we changed the storage from a list to a hash map? How would the time complexity change?

Practice

1. What is the main benefit of using persisted queries in GraphQL?

easy

A. The server stores user credentials for faster login.

B. Clients send only a unique ID instead of the full query, saving bandwidth.

C. Queries are automatically optimized by the client.

D. It allows clients to write queries without validation.

Solution

1. Step 1: Understand what persisted queries do

   Persisted queries store the full GraphQL query on the server with a unique ID.

2. Step 2: Identify the client-server interaction

   Clients send only the ID to run the query, reducing the data sent over the network.

3. Final Answer:

   Clients send only a unique ID instead of the full query, saving bandwidth. -> Option B

4. Quick Check:

   Persisted queries reduce data sent = Clients send only a unique ID instead of the full query, saving bandwidth. [OK]

Hint: Persisted queries send IDs, not full queries [OK]

Common Mistakes:

• Thinking clients send full queries every time
• Confusing persisted queries with client-side caching
• Believing persisted queries store user data

2. Which of the following is the correct way to send a persisted query request in GraphQL?

easy

A. { "id": "12345", "variables": { "userId": "1" } }

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

C. { "mutation": "updateUser" }

D. { "headers": { "Authorization": "token" } }

Solution

1. Step 1: Identify the persisted query request format

   Persisted queries send the unique query ID and variables, not the full query string.

2. Step 2: Match the correct JSON structure

   { "id": "12345", "variables": { "userId": "1" } } sends an ID and variables, which is the correct persisted query format.

3. Final Answer:

   { "id": "12345", "variables": { "userId": "1" } } -> Option A

4. Quick Check:

   Persisted query request = ID + variables [OK]

Hint: Persisted queries use ID field, not full query [OK]

Common Mistakes:

• Sending full query instead of ID
• Using mutation key instead of id
• Confusing headers with query payload

3. Given the following persisted query setup, what will the server return when the client sends { "id": "abc123", "variables": { "limit": 5 } } if the query with ID abc123 fetches the first limit users?

Assume the database has 10 users named User1 to User10.

Options:

medium

A. []

B. [{"name": "User6"}, {"name": "User7"}, {"name": "User8"}, {"name": "User9"}, {"name": "User10"}]

C. [{"name": "User1"}, {"name": "User2"}, {"name": "User3"}, {"name": "User4"}, {"name": "User5"}]

D. Error: Query ID not found

Solution

1. Step 1: Understand the query and variables

   The query with ID 'abc123' fetches users limited by the 'limit' variable, which is 5 here.

2. Step 2: Determine the expected result

   Since the database has users User1 to User10, fetching the first 5 returns User1 to User5.

3. Final Answer:

   [{"name": "User1"}, {"name": "User2"}, {"name": "User3"}, {"name": "User4"}, {"name": "User5"}] -> Option C

4. Quick Check:

   Limit 5 users returns first 5 users [OK]

Hint: Variables control query output; check their values [OK]

Common Mistakes:

• Assuming query returns last users
• Thinking ID is invalid
• Ignoring variables in query

4. A developer tries to use persisted queries but gets an error: Query ID not found. What is the most likely cause?

medium

A. The client sent a query ID that the server does not recognize.

B. The client sent the full query instead of the ID.

C. The server does not support GraphQL.

D. The client forgot to include variables.

Solution

1. Step 1: Analyze the error message

   The error 'Query ID not found' means the server cannot find the query matching the sent ID.

2. Step 2: Identify the cause

   This usually happens if the client sends an ID that was never registered or stored on the server.

3. Final Answer:

   The client sent a query ID that the server does not recognize. -> Option A

4. Quick Check:

   Unknown query ID causes 'Query ID not found' error [OK]

Hint: Check if query ID is registered on server [OK]

Common Mistakes:

• Assuming full query sent causes this error
• Blaming missing variables for this error
• Thinking server lacks GraphQL support

5. You want to secure your GraphQL API by allowing only persisted queries. Which approach best achieves this?

hard

A. Disable persisted queries and use API keys instead.

B. Allow all queries but log those without IDs for review.

C. Require clients to send full queries and IDs together.

D. Reject any request that does not include a valid persisted query ID.

Solution

1. Step 1: Understand API security with persisted queries

   Allowing only persisted queries means the server accepts requests only if they have a valid stored query ID.

2. Step 2: Identify the best enforcement method

   Rejecting requests without valid IDs ensures only approved queries run, improving security.

3. Final Answer:

   Reject any request that does not include a valid persisted query ID. -> Option D

4. Quick Check:

   Only accept valid persisted query IDs to secure API [OK]

Hint: Block requests missing valid persisted query IDs [OK]

Common Mistakes:

• Allowing all queries weakens security
• Sending full queries defeats persisted query purpose
• Relying only on API keys without query control