Table-level permissions in PostgreSQL - Time & Space Complexity

Time Complexity: Table-level permissions

O(n)

Understanding Time Complexity

We want to understand how the time it takes to check table-level permissions changes as the number of tables grows.

How does the system handle permission checks when many tables exist?

Scenario Under Consideration

Analyze the time complexity of this permission check query.

-- Check if user has SELECT permission on a table
SELECT has_table_privilege('username', 'schema.table_name', 'SELECT');

-- Or list all tables user can SELECT from
SELECT tablename
FROM pg_tables
WHERE has_table_privilege('username', schemaname || '.' || tablename, 'SELECT');
    

This code checks if a user has SELECT permission on one or many tables.

Identify Repeating Operations

Look for repeated checks or loops.

• Primary operation: Checking permission for each table using has_table_privilege.
• How many times: Once per table when listing all tables.

How Execution Grows With Input

As the number of tables grows, the system checks permissions for each table separately.

Input Size (n)	Approx. Operations
10	10 permission checks
100	100 permission checks
1000	1000 permission checks

Pattern observation: The number of permission checks grows directly with the number of tables.

Final Time Complexity

Time Complexity: O(n)

This means the time to check permissions grows in a straight line as the number of tables increases.

Common Mistake

[X] Wrong: "Checking permissions on many tables happens instantly regardless of how many tables exist."

[OK] Correct: Each table requires a separate permission check, so more tables mean more work and more time.

Interview Connect

Understanding how permission checks scale helps you design systems that stay fast even as data grows.

Self-Check

"What if we cached permission results for tables? How would the time complexity change when checking permissions repeatedly?"

Practice

1. What does the GRANT SELECT ON table_name TO user_name; command do in PostgreSQL?

easy

A. Removes all permissions from the user on the specified table.

B. Allows the user to delete data from the specified table.

C. Creates a new table with the given name.

D. Allows the user to read data from the specified table.

Solution

1. Step 1: Understand the GRANT command

   The GRANT command is used to give specific permissions to users on database objects like tables.

2. Step 2: Identify the permission type SELECT

   SELECT permission allows reading data from the table but not modifying it.

3. Final Answer:

   Allows the user to read data from the specified table. -> Option D

4. Quick Check:

   GRANT SELECT = read permission [OK]

Hint: GRANT SELECT means read access only [OK]

Common Mistakes:

• Confusing SELECT with DELETE permission
• Thinking GRANT creates tables
• Mixing GRANT with REVOKE commands

2. Which of the following is the correct syntax to revoke INSERT permission on a table named employees from user john?

easy

A. REVOKE INSERT TO john ON employees;

B. REVOKE ON employees INSERT FROM john;

C. REVOKE INSERT ON employees FROM john;

D. REVOKE INSERT FROM john ON employees;

Solution

1. Step 1: Recall REVOKE syntax

   The correct syntax is REVOKE permission ON table FROM user;

2. Step 2: Match syntax with options

   REVOKE INSERT ON employees FROM john; matches the correct order: REVOKE INSERT ON employees FROM john;

3. Final Answer:

   REVOKE INSERT ON employees FROM john; -> Option C

4. Quick Check:

   REVOKE permission ON table FROM user [OK]

Hint: REVOKE syntax: REVOKE permission ON table FROM user [OK]

Common Mistakes:

• Swapping ON and FROM keywords
• Using TO instead of FROM
• Incorrect order of clauses

3. Given the commands:

GRANT SELECT ON orders TO alice;
GRANT INSERT ON orders TO bob;
REVOKE SELECT ON orders FROM alice;

Which of the following is true about user permissions on the orders table?

medium

A. Alice cannot read data; Bob can insert data.

B. Alice can read and insert data; Bob can only insert data.

C. Alice can read data; Bob cannot insert data.

D. Both Alice and Bob have no permissions on the table.

Solution

1. Step 1: Analyze granted permissions

   Alice was granted SELECT (read) permission, Bob was granted INSERT permission.

2. Step 2: Analyze revoked permissions

   Alice's SELECT permission was revoked, so she no longer can read data.

3. Final Answer:

   Alice cannot read data; Bob can insert data. -> Option A

4. Quick Check:

   Revoked SELECT removes read access [OK]

Hint: Revoking removes permission even if previously granted [OK]

Common Mistakes:

• Assuming revoked permission still applies
• Confusing INSERT with SELECT
• Thinking REVOKE affects other users

4. Consider this command:

GRANT UPDATE ON customers TO ;

What is the error in this command?

medium

A. Missing user name after TO keyword.

B. UPDATE is not a valid permission.

C. Table name is missing after ON keyword.

D. GRANT cannot be used for UPDATE permission.

Solution

1. Step 1: Check syntax completeness

   The command ends with TO but does not specify a user or role name.

2. Step 2: Validate permission and table name

   UPDATE is a valid permission and customers is the table name, so those parts are correct.

3. Final Answer:

   Missing user name after TO keyword. -> Option A

4. Quick Check:

   GRANT requires user after TO [OK]

Hint: Always specify user after TO in GRANT [OK]

Common Mistakes:

• Leaving user name blank after TO
• Confusing permission names
• Omitting table name

5. You want to allow user carol to read and insert data into the products table but prevent her from deleting or updating any data. Which commands should you use?

hard

A. GRANT ALL ON products TO carol;
REVOKE DELETE, UPDATE ON products FROM carol;

B. GRANT SELECT, INSERT ON products TO carol;
REVOKE DELETE, UPDATE ON products FROM carol;

C. GRANT SELECT, INSERT, DELETE ON products TO carol;

D. GRANT SELECT ON products TO carol;
GRANT INSERT ON products TO carol;

Solution

1. Step 1: Grant only SELECT and INSERT permissions

   To allow reading and inserting, grant SELECT and INSERT on products to carol.

2. Step 2: Revoke DELETE and UPDATE permissions

   To prevent deleting or updating, explicitly revoke DELETE and UPDATE permissions if previously granted.

3. Final Answer:

   GRANT SELECT, INSERT ON products TO carol; REVOKE DELETE, UPDATE ON products FROM carol; -> Option B

4. Quick Check:

   Grant needed permissions, revoke unwanted ones [OK]

Hint: Grant needed permissions, revoke unwanted explicitly [OK]

Common Mistakes:

• Granting ALL permissions instead of specific ones
• Not revoking unwanted permissions
• Granting DELETE or UPDATE by mistake