Column-level permissions in PostgreSQL - Time & Space Complexity

Time Complexity: Column-level permissions

O(n)

Understanding Time Complexity

When checking column-level permissions in a database, we want to know how the time to verify access grows as the number of columns or users increases.

We ask: How does permission checking scale when more columns or users are involved?

Scenario Under Consideration

Analyze the time complexity of this permission check query.

-- Check if user has access to a specific column
SELECT has_column_privilege('username', 'table_name', 'column_name', 'SELECT');

-- Or check permissions for multiple columns
SELECT column_name
FROM information_schema.columns
WHERE table_name = 'table_name'
  AND has_column_privilege('username', 'table_name', column_name, 'SELECT');
    

This code checks if a user can select specific columns in a table, either one column or multiple columns.

Identify Repeating Operations

Look for repeated checks or loops.

• Primary operation: Checking permission for each column.
• How many times: Once per column when checking multiple columns.

How Execution Grows With Input

As the number of columns grows, the permission check runs once per column.

Input Size (n columns)	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 columns.

Final Time Complexity

Time Complexity: O(n)

This means the time to check permissions grows linearly with the number of columns checked.

Common Mistake

[X] Wrong: "Checking permissions for multiple columns is done all at once, so time stays the same no matter how many columns there are."

[OK] Correct: Each column's permission is checked separately, so more columns mean more checks and more time.

Interview Connect

Understanding how permission checks scale helps you design efficient security in databases and shows you can think about performance in real systems.

Self-Check

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

Practice

1. What does column-level permission in PostgreSQL control?

easy

A. Access to server configuration files

B. Access to entire tables only

C. Access to database schemas

D. Access to specific columns in a table

Solution

1. Step 1: Understand the concept of column-level permissions

   Column-level permissions allow control over which columns a user can see or modify in a table.

2. Step 2: Compare with other access types

   Other options refer to broader or unrelated access controls, not specific columns.

3. Final Answer:

   Access to specific columns in a table -> Option D

4. Quick Check:

   Column-level permission = Access to specific columns [OK]

Hint: Column-level means controlling access per column, not whole table [OK]

Common Mistakes:

• Confusing column-level with table-level permissions
• Thinking it controls schema or server access
• Assuming it controls row-level access

2. Which of the following is the correct syntax to grant SELECT permission on column email of table users to user alice?

easy

A. GRANT SELECT(email) ON users TO alice;

B. GRANT SELECT ON users TO alice(email);

C. GRANT SELECT ON users(email) TO alice;

D. GRANT SELECT TO alice ON users(email);

Solution

1. Step 1: Recall PostgreSQL syntax for column-level GRANT

   The correct syntax is GRANT SELECT (email) ON users TO alice; (parentheses after privilege).

2. Step 2: Match options with syntax

   GRANT SELECT(email) ON users TO alice; matches the correct syntax exactly, others misplace keywords or parentheses.

3. Final Answer:

   GRANT SELECT(email) ON users TO alice; -> Option A

4. Quick Check:

   GRANT SELECT(column) ON table TO user [OK]

Hint: GRANT SELECT(column) ON table TO user is correct syntax [OK]

Common Mistakes:

• Placing column name after TO user
• Putting column inside ON table()
• Misordering keywords in the statement

3. Given the table employees(id, name, salary), if user bob has SELECT permission only on id and name, what will be the result of SELECT * FROM employees; executed by bob?

medium

A. Only id and name columns with data, salary as NULL

B. All columns with actual data

C. Error: permission denied

D. Only salary column with data

Solution

1. Step 1: Understand column-level permission effect on SELECT *

   User can see columns they have permission for; restricted columns appear as NULL.

2. Step 2: Apply to given columns

   Bob has permission on id and name, so salary shows as NULL.

3. Final Answer:

   Only id and name columns with data, salary as NULL -> Option A

4. Quick Check:

   Restricted columns show NULL, not error [OK]

Hint: Restricted columns appear as NULL, not error [OK]

Common Mistakes:

• Expecting a permission denied error
• Assuming all columns show data
• Thinking restricted columns are hidden completely

4. You run the command GRANT SELECT ON employees(name, salary) TO carol; but get a syntax error. What is the likely cause?

medium

A. Column names must be in double quotes

B. Cannot grant permissions on multiple columns at once

C. Column list in parentheses must precede the table name

D. User carol does not exist

Solution

1. Step 1: Check syntax for GRANT with multiple columns

   Correct syntax is GRANT SELECT (name, salary) ON employees TO carol;

2. Step 2: Identify error in command

   Command incorrectly places column list after table name; column list must follow SELECT before ON.

3. Final Answer:

   Column list in parentheses must precede the table name -> Option C

4. Quick Check:

   GRANT SELECT (col1, col2) ON table TO user [OK]

Hint: Columns inside parentheses after SELECT before ON table [OK]

Common Mistakes:

• Placing column list after table name
• Forgetting parentheses around columns
• Assuming multiple columns need separate GRANTs

5. You want to allow user dave to update only the phone column in the contacts table but not others. Which sequence of commands correctly achieves this?

hard

A. GRANT UPDATE(phone) ON contacts TO dave; REVOKE UPDATE ON contacts FROM dave;

B. REVOKE UPDATE ON contacts FROM dave; GRANT UPDATE(phone) ON contacts TO dave;

C. GRANT UPDATE ON contacts TO dave; REVOKE UPDATE(phone) ON contacts FROM dave;

D. GRANT UPDATE(phone) ON contacts TO dave;

Solution

1. Step 1: Remove any existing full UPDATE permission

   First revoke any broad UPDATE permission to avoid conflicts.

2. Step 2: Grant UPDATE permission only on the phone column

   Then grant UPDATE on the specific column to limit access.

3. Final Answer:

   REVOKE UPDATE ON contacts FROM dave; GRANT UPDATE(phone) ON contacts TO dave; -> Option B

4. Quick Check:

   Revoke broad then grant column-level UPDATE [OK]

Hint: Revoke broad permission before granting column-level update [OK]

Common Mistakes:

• Granting column-level without revoking broad permission
• Revoking column-level instead of broad permission
• Assuming single GRANT is enough if broad permission exists