Storage policies for access control in Supabase - Time & Space Complexity

Time Complexity: Storage policies for access control

O(n)

Understanding Time Complexity

When using storage policies for access control, it is important to understand how the number of checks grows as more files or users are involved.

We want to know how the system handles access decisions as the data size increases.

Scenario Under Consideration

Analyze the time complexity of checking access with storage policies.

// Example: Check if a user can read a file
const { data, error } = await supabase
  .storage
  .from('documents')
  .download('file-id');

// Storage policy enforces user_id match for access

This sequence tries to download a file only if the storage policy allows the current user to access it.

Identify Repeating Operations

Look at what happens repeatedly when many files or users are involved.

• Primary operation: Access check against storage policy for each file request.
• How many times: Once per file access attempt.

How Execution Grows With Input

Each file access requires one policy check. If you access more files, the checks increase linearly.

Input Size (n)	Approx. API Calls/Operations
10	10 access checks
100	100 access checks
1000	1000 access checks

Pattern observation: The number of access checks grows directly with the number of file accesses.

Final Time Complexity

Time Complexity: O(n)

This means the time to check access grows in a straight line as you try to access more files.

Common Mistake

[X] Wrong: "Access checks happen once for all files together, so time stays the same no matter how many files."

[OK] Correct: Each file access triggers its own policy check, so more files mean more checks and more time.

Interview Connect

Understanding how access control scales helps you design systems that stay fast and secure as they grow.

Self-Check

"What if storage policies cached user permissions? How would that affect the time complexity of access checks?"

Practice

1. What is the main purpose of storage policies in Supabase?

easy

A. To increase storage space automatically

B. To speed up file uploads

C. To control who can read, upload, or delete files

D. To backup files to another server

Solution

1. Step 1: Understand storage policies

   Storage policies define rules about who can access or change files.

2. Step 2: Identify the main goal

   The main goal is to control access like reading, uploading, or deleting files to keep data safe.

3. Final Answer:

   To control who can read, upload, or delete files -> Option C

4. Quick Check:

   Storage policies = Access control [OK]

Hint: Storage policies = who can do what with files [OK]

Common Mistakes:

• Thinking policies increase storage size
• Confusing policies with backup features
• Believing policies speed up uploads

2. Which of the following is the correct syntax to allow only authenticated users to read files in Supabase storage policy?

easy

A. allow read if user.is_authenticated

B. using (auth.uid() != null)

C. check auth.user == true

D. permit read where auth.id exists

Solution

1. Step 1: Recall Supabase policy syntax

   Supabase uses SQL-like expressions with functions like auth.uid() to check user identity.

2. Step 2: Identify correct condition for authenticated users

   auth.uid() returns user ID if logged in, so checking auth.uid() != null allows only authenticated users.

3. Final Answer:

   using (auth.uid() != null) -> Option B

4. Quick Check:

   auth.uid() != null = authenticated user check [OK]

Hint: Use auth.uid() to check if user is logged in [OK]

Common Mistakes:

• Using incorrect property names like user.is_authenticated
• Writing non-SQL syntax like permit or allow keywords
• Checking auth.id instead of auth.uid()

3. Given this storage policy snippet:

using (bucket_id = 'public' OR auth.uid() = owner_id)

What does this policy allow?

medium

A. Anyone can access files in the 'public' bucket or owners can access their files

B. Only owners can access files in any bucket

C. Only users with bucket_id 'public' can access files

D. No one can access files unless they are owners

Solution

1. Step 1: Analyze the condition

   The condition allows access if bucket_id equals 'public' OR if the user's ID matches owner_id.

2. Step 2: Interpret the meaning

   This means anyone can access files in the 'public' bucket, and owners can access their own files in any bucket.

3. Final Answer:

   Anyone can access files in the 'public' bucket or owners can access their files -> Option A

4. Quick Check:

   bucket_id='public' OR auth.uid()=owner_id = public or owner access [OK]

Hint: OR means either public bucket or owner access allowed [OK]

Common Mistakes:

• Thinking only owners have access
• Assuming bucket_id restricts all access
• Ignoring the OR condition

4. You wrote this policy:

using (auth.uid() = owner_id AND bucket_id = 'private')

But users report they cannot read their private files. What is the likely problem?

medium

A. The policy blocks access because AND requires both conditions, but bucket_id might differ

B. auth.uid() is not a valid function

C. owner_id should be replaced with user_id

D. Policies cannot use AND conditions

Solution

1. Step 1: Understand the AND condition

   AND means both auth.uid() = owner_id and bucket_id = 'private' must be true for access.

2. Step 2: Identify why access fails

   If bucket_id is not exactly 'private' for some files, even owners cannot access them, causing the problem.

3. Final Answer:

   The policy blocks access because AND requires both conditions, but bucket_id might differ -> Option A

4. Quick Check:

   AND requires both true, bucket_id mismatch blocks access [OK]

Hint: AND means all conditions must match exactly [OK]

Common Mistakes:

• Thinking auth.uid() is invalid
• Assuming owner_id is wrong field
• Believing AND is not allowed in policies

5. You want to create a policy that allows users to delete only files they own in any bucket except the 'public' bucket. Which policy condition is correct?

hard

A. using (auth.uid() != owner_id AND bucket_id = 'public')

B. using (auth.uid() = owner_id OR bucket_id != 'public')

C. using (auth.uid() = owner_id AND bucket_id = 'public')

D. using (auth.uid() = owner_id AND bucket_id != 'public')

Solution

1. Step 1: Define the requirements

   Users can delete files only if they own them and the file is not in the 'public' bucket.

2. Step 2: Match the condition

   auth.uid() = owner_id ensures ownership; bucket_id != 'public' excludes the public bucket.

3. Final Answer:

   using (auth.uid() = owner_id AND bucket_id != 'public') -> Option D

4. Quick Check:

   Owner AND not public bucket = correct delete policy [OK]

Hint: Use AND to combine ownership and bucket exclusion [OK]

Common Mistakes:

• Using OR instead of AND allowing wrong deletes
• Checking for equality with 'public' instead of exclusion
• Mixing up != and = operators