Calling functions via RPC in Supabase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What is the output of this Supabase RPC call?

You have a Postgres function named get_user_count that returns the total number of users in the users table. You call it via Supabase RPC like this:

const { data, error } = await supabase.rpc('get_user_count');
console.log(data);

If the users table has 42 rows, what will console.log(data) output?

Supabase

const { data, error } = await supabase.rpc('get_user_count');
console.log(data);

A [42]

B { count: 42 }

C 42

D null

💡 Hint

Supabase RPC returns an array of results by default.

Supabase RPC calls return an array of rows from the Postgres function. If the function returns a single scalar value, it is wrapped in an array. So the output is [42].

❓ Configuration

intermediate

Which option correctly calls a Postgres function with parameters via Supabase RPC?

You have a Postgres function get_user_by_email(email text) that returns user info for a given email. Which of these Supabase RPC calls correctly passes the email parameter?

A await supabase.rpc('get_user_by_email', email='[e-mail]');

B await supabase.rpc('get_user_by_email', ['[e-mail]']);

C await supabase.rpc('get_user_by_email', { email: '[e-mail]' });

D await supabase.rpc('get_user_by_email', '[e-mail]');

💡 Hint

Supabase RPC expects parameters as an object with keys matching function arguments.

Supabase RPC requires parameters as an object where keys match the Postgres function's argument names. Option C correctly passes { email: '[e-mail]' }.

❓ Architecture

advanced

What is the best way to secure a Postgres function exposed via Supabase RPC?

You want to expose a Postgres function via Supabase RPC but restrict access only to authenticated users. Which approach ensures this?

A Set the function's security to 'SECURITY DEFINER' and check current_user inside the function.

B Disable anonymous access in Supabase and allow public access to the function.

C Make the function 'VOLATILE' so it runs only for authenticated users.

D Use Supabase Row Level Security (RLS) policies on the underlying tables and require JWT authentication.

💡 Hint

Supabase uses RLS policies to control data access per user.

Supabase enforces security mainly via Row Level Security policies on tables. Functions should rely on RLS and JWT authentication to restrict access properly.

✅ Best Practice

advanced

Which practice improves performance when calling Postgres functions via Supabase RPC?

You have a function that returns a large dataset. Which practice helps reduce latency and improve performance when calling it via Supabase RPC?

A Call the function multiple times with different parameters to get smaller chunks.

B Use LIMIT and OFFSET parameters in the function to paginate results.

C Return all columns and rows from the function and filter results client-side.

D Disable caching on Supabase to always get fresh data.

💡 Hint

Fetching smaller chunks reduces data transfer and speeds up response.

Using LIMIT and OFFSET in the function to paginate results reduces data size per call, improving performance and user experience.

🧠 Conceptual

expert

What error occurs if you call a non-existent Postgres function via Supabase RPC?

You call await supabase.rpc('non_existing_function'); but the function does not exist in the database. What error will Supabase return?

Supabase

await supabase.rpc('non_existing_function');

A {"code":"42883","message":"function non_existing_function() does not exist"}

B {"code":"23505","message":"unique_violation"}

C {"code":"42601","message":"syntax error"}

D {"code":"42501","message":"permission denied"}

💡 Hint

Error code 42883 means function not found in Postgres.

Postgres error code 42883 indicates a missing function. Supabase returns this error when the RPC function does not exist.

Practice

1. What does supabase.rpc() do in a Supabase app?

easy

A. Creates a new table in the database

B. Uploads files to storage

C. Fetches user authentication details

D. Calls a custom database function remotely

Solution

1. Step 1: Understand the purpose of supabase.rpc()

   This method is designed to call remote procedure calls (RPC), which are custom functions defined in the database.

2. Step 2: Identify what supabase.rpc() does in the app context

   It runs these custom functions securely from the app, allowing logic to stay in the database.

3. Final Answer:

   Calls a custom database function remotely -> Option D

4. Quick Check:

   RPC calls = custom database functions [OK]

Hint: RPC means running database functions remotely [OK]

Common Mistakes:

• Confusing RPC with file uploads
• Thinking it creates tables
• Assuming it handles user login

2. Which is the correct way to call a function named get_user_data with parameter { user_id: 5 } using Supabase RPC?

easy

A. supabase.rpc('get_user_data', { user_id: 5 })

B. supabase.call('get_user_data', 5)

C. supabase.function('get_user_data', { id: 5 })

D. supabase.execute('get_user_data', user_id=5)

Solution

1. Step 1: Recall the syntax of supabase.rpc()

   The method takes the function name as a string and an object with parameters.

2. Step 2: Match the correct syntax with the options

   supabase.rpc('get_user_data', { user_id: 5 }) correctly uses supabase.rpc('function_name', { params }) format.

3. Final Answer:

   supabase.rpc('get_user_data', { user_id: 5 }) -> Option A

4. Quick Check:

   Correct RPC call syntax = supabase.rpc('get_user_data', { user_id: 5 }) [OK]

Hint: Use rpc('function', {params}) format [OK]

Common Mistakes:

• Using wrong method names like call or execute
• Passing parameters as separate arguments
• Using incorrect parameter keys

3. Given this code snippet:

const { data, error } = await supabase.rpc('multiply', { a: 3, b: 4 });
console.log(data);

What will be printed if the multiply function returns the product of a and b?

medium

A. 7

B. 12

C. undefined

D. Error

Solution

1. Step 1: Understand the function call and parameters

   The function multiply is called with a=3 and b=4.

2. Step 2: Calculate the expected return value

   Since the function returns the product, 3 * 4 = 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   3 times 4 = 12 [OK]

Hint: Multiply parameters to find output [OK]

Common Mistakes:

• Adding instead of multiplying
• Assuming no return value
• Confusing data with error

4. You wrote this code:

const { data, error } = await supabase.rpc('get_user', { user_id: 10 });
console.log(data);

But you get an error saying the function does not exist. What is the likely cause?

medium

A. Parameters must be passed as an array, not an object

B. You forgot to import supabase client

C. The function name is misspelled or missing in the database

D. RPC calls do not support parameters

Solution

1. Step 1: Analyze the error message

   The error says the function does not exist, indicating a problem with the function name or its presence in the database.

2. Step 2: Check common causes for missing function errors

   Usually, the function is either misspelled or not created in the database schema.

3. Final Answer:

   The function name is misspelled or missing in the database -> Option C

4. Quick Check:

   Function missing error = wrong or absent function [OK]

Hint: Check function spelling and existence in DB [OK]

Common Mistakes:

• Passing parameters incorrectly
• Assuming import errors cause this
• Thinking RPC disallows parameters

5. You want to create a Supabase RPC call to get the total sales for a given store_id. The database function is named total_sales and takes store_id as input. Which code snippet correctly calls this function and handles errors?

hard

A. const { data, error } = await supabase.rpc('total_sales', { store_id }); if (error) console.error(error); else console.log(data);

B. const result = supabase.call('total_sales', store_id); console.log(result);

C. const { data } = await supabase.rpc('total_sales', store_id); console.log(data);

D. const { data, error } = supabase.rpc('total_sales', { store: store_id }); if (error) throw error; else console.log(data);

Solution

1. Step 1: Identify correct RPC call syntax with parameters

   The function name is 'total_sales' and parameters must be passed as an object with key matching the function input, here { store_id }.

2. Step 2: Check error handling and async usage

   const { data, error } = await supabase.rpc('total_sales', { store_id }); if (error) console.error(error); else console.log(data); uses await, destructures data and error, and handles error properly with console.error.

3. Final Answer:

   const { data, error } = await supabase.rpc('total_sales', { store_id }); if (error) console.error(error); else console.log(data); -> Option A

4. Quick Check:

   Correct RPC call with error check = const { data, error } = await supabase.rpc('total_sales', { store_id }); if (error) console.error(error); else console.log(data); [OK]

Hint: Use await, pass params as object, check error [OK]

Common Mistakes:

• Passing parameters incorrectly (not as object)
• Ignoring error handling
• Using wrong method names like call