Table relationships in Supabase - Time & Space Complexity

Time Complexity: Table relationships

O(n)

Understanding Time Complexity

When working with table relationships in Supabase, it's important to understand how the number of related records affects the time it takes to fetch data.

We want to know: how does the time to get related data grow as the amount of related data grows?

Scenario Under Consideration

Analyze the time complexity of fetching a user and their related orders.

const { data, error } = await supabase
  .from('users')
  .select('id, name, orders(*)')
  .eq('id', userId)

This code fetches one user by ID and includes all their related orders in the result.

Identify Repeating Operations

Here are the key operations that repeat:

• Primary operation: Fetching each related order record from the orders table.
• How many times: Once for each order linked to the user.

How Execution Grows With Input

As the number of orders for a user grows, the time to fetch all orders grows roughly the same way.

Input Size (n)	Approx. Api Calls/Operations
10 orders	Fetching 1 user + 10 orders
100 orders	Fetching 1 user + 100 orders
1000 orders	Fetching 1 user + 1000 orders

Pattern observation: The time grows linearly with the number of related orders.

Final Time Complexity

Time Complexity: O(n)

This means the time to fetch data grows directly in proportion to the number of related records.

Common Mistake

[X] Wrong: "Fetching related data is always a single fast operation regardless of size."

[OK] Correct: Each related record adds to the total data fetched, so more related records mean more time.

Interview Connect

Understanding how related data fetching scales helps you design efficient queries and explain your reasoning clearly in real-world situations.

Self-Check

"What if we only fetched the count of related orders instead of all order details? How would the time complexity change?"

Practice

1. What is the main purpose of creating table relationships in Supabase?

easy

A. To store files in the database

B. To connect data across tables using keys

C. To increase the size of the database

D. To change the database theme

Solution

1. Step 1: Understand table relationships

   Table relationships link data in different tables using keys to keep data connected.

2. Step 2: Identify the purpose

   This linking helps to find related data easily, like joining customer info with orders.

3. Final Answer:

   To connect data across tables using keys -> Option B

4. Quick Check:

   Table relationships = Connect data [OK]

Hint: Relationships link tables by keys for easy data access [OK]

Common Mistakes:

• Thinking relationships store files
• Believing relationships increase database size
• Confusing relationships with UI themes

2. Which SQL keyword is used in Supabase to create a foreign key relationship between tables?

easy

A. REFERENCES

B. JOIN

C. SELECT

D. INSERT

Solution

1. Step 1: Identify the keyword for foreign keys

   In SQL, the REFERENCES keyword defines a foreign key that links one table's column to another's primary key.

2. Step 2: Match with Supabase usage

   Supabase uses standard SQL, so REFERENCES is used to create safe links between tables.

3. Final Answer:

   REFERENCES -> Option A

4. Quick Check:

   Foreign key = REFERENCES [OK]

Hint: Use REFERENCES to link tables with foreign keys [OK]

Common Mistakes:

• Using JOIN to create relationships (JOIN reads data, doesn't create keys)
• Confusing SELECT or INSERT with key creation

3. Given these two tables:
CREATE TABLE authors (id SERIAL PRIMARY KEY, name TEXT);
CREATE TABLE books (id SERIAL PRIMARY KEY, title TEXT, author_id INT REFERENCES authors(id));
What will this query return?
SELECT books.title, authors.name FROM books JOIN authors ON books.author_id = authors.id;

medium

A. A list of books without author names

B. A list of authors without book titles

C. An error because of missing foreign key

D. A list of book titles with their author's names

Solution

1. Step 1: Understand the table relationship

   The books table has a foreign key author_id linking to authors.id, connecting each book to its author.

2. Step 2: Analyze the JOIN query

   The query joins books and authors on matching author_id and id, selecting book titles and author names together.

3. Final Answer:

   A list of book titles with their author's names -> Option D

4. Quick Check:

   JOIN on foreign key returns related data [OK]

Hint: JOIN returns combined rows matching foreign keys [OK]

Common Mistakes:

• Expecting authors without books from this JOIN
• Thinking query causes error due to foreign key
• Assuming books without author names appear

4. You wrote this table creation:
CREATE TABLE orders (id SERIAL PRIMARY KEY, customer_id INT, FOREIGN KEY customer_id REFERENCES customers(id));
Why does this cause an error in Supabase?

medium

A. FOREIGN KEY syntax is incorrect; it needs parentheses around the column

B. customer_id must be TEXT, not INT

C. PRIMARY KEY cannot be SERIAL

D. REFERENCES keyword is not supported

Solution

1. Step 1: Check FOREIGN KEY syntax

   The FOREIGN KEY clause requires parentheses around the column name, like FOREIGN KEY (customer_id).

2. Step 2: Confirm other parts

   customer_id as INT is valid, SERIAL primary key is valid, and REFERENCES is supported in Supabase.

3. Final Answer:

   FOREIGN KEY syntax is incorrect; it needs parentheses around the column -> Option A

4. Quick Check:

   FOREIGN KEY syntax = parentheses needed [OK]

Hint: Put column names in parentheses after FOREIGN KEY [OK]

Common Mistakes:

• Omitting parentheses around foreign key column
• Changing data type unnecessarily
• Thinking REFERENCES is unsupported

5. You want to create a many-to-many relationship between students and courses tables in Supabase. Which setup is best?

hard

A. Add a foreign key column in students pointing to courses

B. Add a foreign key column in courses pointing to students

C. Create a join table with student_id and course_id as foreign keys

D. Merge students and courses into one table

Solution

1. Step 1: Understand many-to-many relationships

   Many-to-many means each student can take many courses and each course can have many students.

2. Step 2: Choose the correct design

   Neither students nor courses alone can hold all links. A join table with student_id and course_id foreign keys connects them properly.

3. Final Answer:

   Create a join table with student_id and course_id as foreign keys -> Option C

4. Quick Check:

   Many-to-many = join table with two foreign keys [OK]

Hint: Use a join table for many-to-many links [OK]

Common Mistakes:

• Trying to store many-to-many in one table column
• Merging unrelated tables
• Using one foreign key only