Schema-level access control in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this schema access query?

Consider a PostgreSQL database with a schema named sales. The user alice has been granted USAGE on the sales schema but no other privileges. What will be the result of the following query when run by alice?

SELECT * FROM sales.orders;

PostgreSQL

GRANT USAGE ON SCHEMA sales TO alice;
-- alice runs:
SELECT * FROM sales.orders;

A ERROR: permission denied for relation orders

B Returns all rows from sales.orders table

C ERROR: schema sales does not exist

D Returns no rows (empty result set)

💡 Hint

USAGE on a schema allows access to the schema itself but not to the tables inside it.

Granting USAGE on a schema allows the user to access objects in the schema if they have privileges on those objects. Since alice has no SELECT privilege on sales.orders, the query fails with a permission denied error.

🧠 Conceptual

intermediate

Which privilege is required to create tables in a schema?

In PostgreSQL, to create new tables inside an existing schema, which privilege must a user have on that schema?

A USAGE

B INSERT

C SELECT

D CREATE

💡 Hint

Think about what permission allows adding new objects inside a schema.

The CREATE privilege on a schema allows a user to create new objects such as tables within that schema. USAGE alone does not allow creation.

📝 Syntax

advanced

Identify the correct syntax to grant SELECT on all tables in a schema

Which of the following commands correctly grants SELECT privilege on all current tables in the inventory schema to user bob?

A GRANT SELECT ON SCHEMA inventory TO bob;

B GRANT SELECT ON ALL TABLES IN SCHEMA inventory TO bob;

C GRANT SELECT ON inventory.* TO bob;

D GRANT SELECT ON ALL TABLES inventory TO bob;

💡 Hint

Look for the exact phrase used in PostgreSQL to grant privileges on all tables in a schema.

The correct syntax is GRANT SELECT ON ALL TABLES IN SCHEMA inventory TO bob;. Other options are either invalid or grant privileges on the schema itself, not the tables.

❓ optimization

advanced

Best way to allow read access to all future tables in a schema

You want user carol to have SELECT access on all current and future tables in the reports schema. Which approach achieves this with minimal ongoing maintenance?

A Grant USAGE on the schema and ALTER DEFAULT PRIVILEGES to grant SELECT on future tables to carol.

B Grant SELECT on all current tables and repeat the grant manually for new tables.

C Grant CREATE on the schema to carol so she can create tables and access them.

D Grant SELECT on the schema itself.

💡 Hint

Think about how to automate privileges for tables created later.

Granting USAGE on the schema plus setting ALTER DEFAULT PRIVILEGES to grant SELECT on future tables ensures carol automatically gets access to new tables without manual grants.

🔧 Debug

expert

Why does this schema-level grant not allow table access?

User dave runs the following commands:

GRANT USAGE ON SCHEMA analytics TO dave;
SELECT * FROM analytics.events;

But he gets an error: permission denied for relation events. Why?

A Dave needs CONNECT privilege on the database first.

B The schema <code>analytics</code> does not exist.

C USAGE on schema does not grant any privileges on tables inside it.

D The table <code>events</code> is empty, so no rows are returned.

💡 Hint

Consider what USAGE on a schema actually allows.

USAGE on a schema allows access to the schema namespace but does not grant any rights on the tables inside. Dave needs explicit SELECT privilege on analytics.events to query it.

Practice

1. What does the USAGE privilege on a schema in PostgreSQL allow a user to do?

easy

A. Access objects within the schema without creating new ones

B. Create new tables and objects inside the schema

C. Delete the schema entirely

D. Modify data in tables outside the schema

Solution

1. Step 1: Understand USAGE privilege meaning

   The USAGE privilege allows a user to access objects inside the schema, such as selecting data from tables, but does not allow creating new objects.

2. Step 2: Differentiate from CREATE privilege

   The CREATE privilege is needed to add new tables or other objects. USAGE alone does not grant this ability.

3. Final Answer:

   Access objects within the schema without creating new ones -> Option A

4. Quick Check:

   USAGE = access only [OK]

Hint: USAGE lets you use, CREATE lets you add [OK]

Common Mistakes:

• Confusing USAGE with CREATE privilege
• Thinking USAGE allows schema deletion
• Assuming USAGE grants data modification outside schema

2. Which of the following is the correct syntax to grant CREATE privilege on a schema named sales to user alice?

easy

A. GRANT CREATE ON sales TO alice;

B. GRANT CREATE TO alice ON SCHEMA sales;

C. GRANT CREATE ON SCHEMA sales TO alice;

D. GRANT CREATE ON DATABASE sales TO alice;

Solution

1. Step 1: Identify correct GRANT syntax for schema

   In PostgreSQL, to grant privileges on a schema, the syntax is: GRANT privilege ON SCHEMA schema_name TO user;

2. Step 2: Match syntax with options

   GRANT CREATE ON SCHEMA sales TO alice; matches this syntax exactly: GRANT CREATE ON SCHEMA sales TO alice;

3. Final Answer:

   GRANT CREATE ON SCHEMA sales TO alice; -> Option C

4. Quick Check:

   GRANT ... ON SCHEMA ... TO ... [OK]

Hint: Use 'ON SCHEMA' when granting schema privileges [OK]

Common Mistakes:

• Omitting 'SCHEMA' keyword
• Using 'ON DATABASE' instead of 'ON SCHEMA'
• Placing TO clause incorrectly

3. Given the commands below, what will be the result of SELECT * FROM sales.orders; when run by user bob?

GRANT USAGE ON SCHEMA sales TO bob;
REVOKE CREATE ON SCHEMA sales FROM bob;

medium

A. Query runs but returns no rows

B. Error: permission denied for schema sales

C. Error: relation sales.orders does not exist

D. Query runs successfully and returns rows from sales.orders

Solution

1. Step 1: Analyze granted privileges

   User bob has USAGE on schema sales, so can access objects inside it. CREATE privilege is revoked, so bob cannot create new objects but can read existing ones.

2. Step 2: Understand effect on SELECT query

   Since bob has USAGE, SELECT on sales.orders will work if bob has SELECT privilege on the table (assumed). The REVOKE of CREATE does not affect SELECT.

3. Final Answer:

   Query runs successfully and returns rows from sales.orders -> Option D

4. Quick Check:

   USAGE allows access, REVOKE CREATE blocks creation only [OK]

Hint: USAGE lets you read; CREATE controls adding objects [OK]

Common Mistakes:

• Confusing CREATE with SELECT privilege
• Assuming REVOKE CREATE blocks all access
• Ignoring USAGE privilege effect

4. You want to allow user carol to create tables in schema inventory, but she gets an error: permission denied for schema inventory. Which command fixes this?

medium

A. GRANT CREATE ON SCHEMA inventory TO carol;

B. GRANT USAGE ON SCHEMA inventory TO carol;

C. GRANT ALL PRIVILEGES ON SCHEMA inventory TO carol;

D. REVOKE USAGE ON SCHEMA inventory FROM carol;

Solution

1. Step 1: Understand error cause

   To create tables, user needs both USAGE and CREATE privileges on the schema. Without USAGE, permission denied error occurs.

2. Step 2: Grant missing privilege

   Granting USAGE on schema inventory to carol allows her to access the schema and create tables if CREATE is already granted.

3. Final Answer:

   GRANT USAGE ON SCHEMA inventory TO carol; -> Option B

4. Quick Check:

   USAGE needed before CREATE works [OK]

Hint: Grant USAGE before CREATE to avoid permission errors [OK]

Common Mistakes:

• Granting CREATE without USAGE privilege
• Revoking instead of granting privileges
• Assuming ALL PRIVILEGES always needed

5. You want to restrict user dave so he can only create objects in schema projects but cannot access any existing objects. Which combination of privileges achieves this?

hard

A. GRANT CREATE ON SCHEMA projects TO dave; REVOKE USAGE ON SCHEMA projects FROM dave;

B. GRANT USAGE ON SCHEMA projects TO dave; REVOKE CREATE ON SCHEMA projects FROM dave;

C. GRANT ALL ON SCHEMA projects TO dave;

D. REVOKE ALL ON SCHEMA projects FROM dave;

Solution

1. Step 1: Understand privilege effects

   CREATE allows adding new objects. USAGE allows accessing existing objects. To restrict access but allow creation, grant CREATE and revoke USAGE.

2. Step 2: Apply correct commands

   GRANT CREATE ON SCHEMA projects TO dave; REVOKE USAGE ON SCHEMA projects FROM dave; grants CREATE and revokes USAGE, so dave can create but not access existing objects.

3. Final Answer:

   GRANT CREATE ON SCHEMA projects TO dave; REVOKE USAGE ON SCHEMA projects FROM dave; -> Option A

4. Quick Check:

   Create without usage blocks access [OK]

Hint: Grant CREATE, revoke USAGE to allow creation only [OK]

Common Mistakes:

• Granting USAGE allows access to existing objects
• Revoking CREATE disables creation
• Granting ALL gives too many rights