Public schema vs custom schemas in PostgreSQL - Performance Comparison

Time Complexity: Public schema vs custom schemas

O(n)

Understanding Time Complexity

When working with PostgreSQL schemas, it's important to understand how accessing tables in different schemas affects query performance.

We want to see how the time to find and use tables changes when using the public schema versus custom schemas.

Scenario Under Consideration

Analyze the time complexity of querying a table in different schemas.

-- Query from public schema
SELECT * FROM public.users WHERE id = 10;

-- Query from custom schema
SELECT * FROM custom_schema.users WHERE id = 10;

-- Setting search path to custom schema
SET search_path TO custom_schema;
SELECT * FROM users WHERE id = 10;
    

This code shows queries accessing the same table name in different schemas and using search_path to simplify access.

Identify Repeating Operations

Look at what happens when PostgreSQL finds the table to run the query.

• Primary operation: Schema lookup to find the table definition.
• How many times: Once per query execution, but can repeat if multiple tables or schema changes occur.

How Execution Grows With Input

Finding a table in a schema is like looking up a name in a small list.

Number of Schemas	Approx. Lookup Steps
1 (public only)	1 step
5 schemas	Up to 5 steps
100 schemas	Up to 100 steps

Pattern observation: The more schemas in the search path, the longer it may take to find the table, growing roughly linearly with the number of schemas.

Final Time Complexity

Time Complexity: O(n)

This means the time to find a table grows linearly with the number of schemas checked in the search path.

Common Mistake

[X] Wrong: "Using the public schema is always faster than custom schemas because it's the default."

[OK] Correct: The speed depends on how many schemas PostgreSQL searches before finding the table, not just which schema it is.

Interview Connect

Understanding how schema search paths affect query time helps you design databases that are easy to maintain and perform well.

Self-Check

"What if we add an index on the schema name lookup? How would that change the time complexity of finding tables?"

Practice

1. What is the role of the public schema in PostgreSQL?

easy

A. It is a special schema that cannot contain tables or other objects.

B. It is the default schema where database objects are created if no schema is specified.

C. It is used only for system tables and cannot be modified by users.

D. It automatically encrypts all data stored within it.

Solution

1. Step 1: Understand the default schema concept

   In PostgreSQL, the public schema is created by default in every new database.

2. Step 2: Identify the role of the public schema

   When you create tables or other objects without specifying a schema, they go into the public schema automatically.

3. Final Answer:

   It is the default schema where database objects are created if no schema is specified. -> Option B

4. Quick Check:

   Default schema = public [OK]

Hint: Remember: public schema is default for new objects [OK]

Common Mistakes:

• Thinking public schema is read-only
• Assuming public schema is for system tables only
• Believing public schema encrypts data automatically

2. Which of the following is the correct way to create a new schema named sales in PostgreSQL?

easy

A. CREATE SCHEMA sales;

B. CREATE DATABASE sales;

C. NEW SCHEMA sales;

D. MAKE SCHEMA sales;

Solution

1. Step 1: Recall the SQL syntax for creating schemas

   The correct SQL command to create a schema is CREATE SCHEMA schema_name;.

2. Step 2: Check each option

   CREATE SCHEMA sales; uses the correct syntax. MAKE SCHEMA sales;, CREATE DATABASE sales;, and NEW SCHEMA sales; are invalid SQL commands for creating schemas.

3. Final Answer:

   CREATE SCHEMA sales; -> Option A

4. Quick Check:

   CREATE SCHEMA = create new schema [OK]

Hint: Use CREATE SCHEMA to make new schemas [OK]

Common Mistakes:

• Using CREATE DATABASE instead of CREATE SCHEMA
• Using non-existent commands like NEW SCHEMA or MAKE SCHEMA
• Forgetting the semicolon at the end

3. Given the following commands:

CREATE SCHEMA marketing;
CREATE TABLE marketing.campaigns (id SERIAL PRIMARY KEY, name TEXT);
SET search_path TO marketing, public;
SELECT * FROM campaigns;

What will be the result of the SELECT query?

medium

A. It will return rows from a campaigns table in the public schema if it exists.

B. It will cause an error because the table name is not schema-qualified.

C. It will return all rows from the marketing.campaigns table.

D. It will return no rows because search_path is set incorrectly.

Solution

1. Step 1: Understand search_path usage

   The search_path is set to look first in marketing schema, then public.

2. Step 2: Identify which table is found

   The table campaigns exists in marketing schema, so the query finds and returns data from marketing.campaigns.

3. Final Answer:

   It will return all rows from the marketing.campaigns table. -> Option C

4. Quick Check:

   search_path order finds marketing first [OK]

Hint: search_path order decides which schema is used first [OK]

Common Mistakes:

• Assuming table must be schema-qualified always
• Thinking query errors without schema prefix
• Ignoring search_path order

4. You run this command:

SELECT * FROM sales.orders;

But get the error: ERROR: schema "sales" does not exist.
What is the most likely cause?

medium

A. The sales schema has not been created yet.

B. The orders table does not exist in the public schema.

C. You forgot to set the search_path to include sales.

D. You need to use double quotes around sales.

Solution

1. Step 1: Analyze the error message

   The error says the schema "sales" does not exist, meaning PostgreSQL cannot find that schema.

2. Step 2: Identify the cause

   This usually means the schema was never created or was dropped, so the database has no knowledge of it.

3. Final Answer:

   The sales schema has not been created yet. -> Option A

4. Quick Check:

   Schema missing causes this error [OK]

Hint: Check if schema exists before querying it [OK]

Common Mistakes:

• Assuming search_path affects schema existence errors
• Thinking table absence causes schema not found error
• Trying to fix with quotes unnecessarily

5. You want to organize your database by creating two schemas: hr and finance. You create tables with the same name employees in both schemas. How can you write a query to select all employees from the finance schema without changing the search_path?

hard

A. SELECT * FROM public.employees WHERE schema = 'finance';

B. SET search_path TO finance; SELECT * FROM employees;

C. SELECT * FROM employees WHERE schema = 'finance';

D. SELECT * FROM finance.employees;

Solution

1. Step 1: Understand schema-qualified table names

   When tables have the same name in different schemas, you can specify the schema explicitly before the table name.

2. Step 2: Identify the correct query

   SELECT * FROM finance.employees; uses finance.employees which directly accesses the table in the finance schema without changing search_path.

3. Final Answer:

   SELECT * FROM finance.employees; -> Option D

4. Quick Check:

   Schema-qualified names access tables directly [OK]

Hint: Use schema.table to access specific schema tables [OK]

Common Mistakes:

• Trying to filter by schema column which doesn't exist
• Changing search_path unnecessarily
• Assuming public schema contains all tables