Join algorithms (nested loop, hash, merge) in PostgreSQL - Mini Project: Build & Apply

Explore Join Algorithms in PostgreSQL

📖 Scenario: You are working with a small bookstore database. The database has two tables: authors and books. Each book has an author_id that links it to an author. You want to learn how to join these tables using different join algorithms in PostgreSQL.

🎯 Goal: Build SQL queries that join the authors and books tables using nested loop join, hash join, and merge join algorithms. You will create the tables, insert data, and write queries that demonstrate each join type.

📋 What You'll Learn

Create authors and books tables with specified columns

Insert exact data into both tables

Write a query that uses a nested loop join

Write a query that uses a hash join

Write a query that uses a merge join

💡 Why This Matters

🌍 Real World

Database administrators and developers often need to understand how different join algorithms work to optimize query performance in real applications.

💼 Career

Knowing how to control and analyze join algorithms is important for roles like database developer, data engineer, and backend developer to write efficient SQL queries.

1

Create tables and insert data

Create a table called authors with columns author_id (integer primary key) and name (text). Create another table called books with columns book_id (integer primary key), title (text), and author_id (integer). Insert these exact rows into authors: (1, 'Jane Austen'), (2, 'Mark Twain'), (3, 'Charles Dickens'). Insert these exact rows into books: (101, 'Pride and Prejudice', 1), (102, 'Adventures of Huckleberry Finn', 2), (103, 'Oliver Twist', 3), (104, 'Emma', 1).

PostgreSQL

-- Create authors and books tables and insert data
-- Your code here

Hint

Use CREATE TABLE statements for both tables. Then use INSERT INTO with multiple rows for inserting data.

2

Enable nested loop join

Set the PostgreSQL configuration to enable only nested loop joins by running SET enable_nestloop = on; and disable hash and merge joins by running SET enable_hashjoin = off; and SET enable_mergejoin = off;.

PostgreSQL

CREATE TABLE authors (
    author_id INTEGER PRIMARY KEY,
    name TEXT
);

CREATE TABLE books (
    book_id INTEGER PRIMARY KEY,
    title TEXT,
    author_id INTEGER
);

INSERT INTO authors (author_id, name) VALUES
(1, 'Jane Austen'),
(2, 'Mark Twain'),
(3, 'Charles Dickens');

INSERT INTO books (book_id, title, author_id) VALUES
(101, 'Pride and Prejudice', 1),
(102, 'Adventures of Huckleberry Finn', 2),
(103, 'Oliver Twist', 3),
(104, 'Emma', 1);

-- Set join algorithm to nested loop only
-- Your code here

Hint

Use SET commands to enable and disable join types.

3

Write a nested loop join query

Write a SQL query that joins authors and books on author_id using an explicit INNER JOIN. Select authors.name and books.title. Use the join algorithm settings from Step 2.

PostgreSQL

CREATE TABLE authors (
    author_id INTEGER PRIMARY KEY,
    name TEXT
);

CREATE TABLE books (
    book_id INTEGER PRIMARY KEY,
    title TEXT,
    author_id INTEGER
);

INSERT INTO authors (author_id, name) VALUES
(1, 'Jane Austen'),
(2, 'Mark Twain'),
(3, 'Charles Dickens');

INSERT INTO books (book_id, title, author_id) VALUES
(101, 'Pride and Prejudice', 1),
(102, 'Adventures of Huckleberry Finn', 2),
(103, 'Oliver Twist', 3),
(104, 'Emma', 1);

SET enable_nestloop = on;
SET enable_hashjoin = off;
SET enable_mergejoin = off;

-- Write a query joining authors and books on author_id
-- Your code here

Hint

Use SELECT with INNER JOIN and the ON clause to join tables.

4

Write hash join and merge join queries

First, set PostgreSQL to enable only hash joins by running SET enable_hashjoin = on;, and disable nested loop and merge joins by running SET enable_nestloop = off; and SET enable_mergejoin = off;. Then write the same join query as Step 3. Next, set PostgreSQL to enable only merge joins by running SET enable_mergejoin = on;, and disable nested loop and hash joins by running SET enable_nestloop = off; and SET enable_hashjoin = off;. Then write the same join query again.

PostgreSQL

CREATE TABLE authors (
    author_id INTEGER PRIMARY KEY,
    name TEXT
);

CREATE TABLE books (
    book_id INTEGER PRIMARY KEY,
    title TEXT,
    author_id INTEGER
);

INSERT INTO authors (author_id, name) VALUES
(1, 'Jane Austen'),
(2, 'Mark Twain'),
(3, 'Charles Dickens');

INSERT INTO books (book_id, title, author_id) VALUES
(101, 'Pride and Prejudice', 1),
(102, 'Adventures of Huckleberry Finn', 2),
(103, 'Oliver Twist', 3),
(104, 'Emma', 1);

SET enable_nestloop = on;
SET enable_hashjoin = off;
SET enable_mergejoin = off;

SELECT authors.name, books.title
FROM authors
INNER JOIN books ON authors.author_id = books.author_id;

-- Now enable hash join only and write the join query
-- Your code here

-- Then enable merge join only and write the join query
-- Your code here

Hint

Use SET commands to switch join algorithms, then write the same join query each time.

Practice

1. Which join algorithm in PostgreSQL is best suited for small tables or when one table is very small compared to the other?

easy

A. Index Join

B. Hash Join

C. Nested Loop Join

D. Merge Join

Solution

1. Step 1: Understand Nested Loop Join usage

   Nested Loop Join works by scanning one table and for each row scanning the other table. It is efficient when one table is small.

2. Step 2: Compare with other joins

   Hash Join is better for large unsorted tables, Merge Join requires sorted inputs. Nested Loop is simplest and best for small tables.

3. Final Answer:

   Nested Loop Join -> Option C

4. Quick Check:

   Small table + Nested Loop Join = best [OK]

Hint: Small table joins usually use Nested Loop Join [OK]

Common Mistakes:

• Confusing Hash Join as best for small tables
• Thinking Merge Join works well without sorted data
• Assuming Index Join is a separate join algorithm

2. Which of the following is the correct syntax to hint PostgreSQL to use a Hash Join in a query?

easy

A. SELECT /*+ HashJoin */ * FROM table1 JOIN table2 ON table1.id = table2.id;

B. SET enable_hashjoin = on; SELECT * FROM table1 JOIN table2 ON table1.id = table2.id;

C. SELECT * FROM table1 HASH JOIN table2 ON table1.id = table2.id;

D. SELECT * FROM table1 JOIN table2 USING HASH(id);

Solution

1. Step 1: Understand PostgreSQL join hints

   PostgreSQL does not support inline join hints like /*+ HashJoin */ or HASH JOIN syntax.

2. Step 2: Use configuration to enable Hash Join

   We can enable or disable join types using SET commands, e.g., SET enable_hashjoin = on; before the query.

3. Final Answer:

   SET enable_hashjoin = on; SELECT ... -> Option B

4. Quick Check:

   PostgreSQL uses SET to enable join types [OK]

Hint: Use SET enable_hashjoin to control hash join usage [OK]

Common Mistakes:

• Using Oracle-style hints like /*+ HashJoin */
• Trying to write HASH JOIN in SQL syntax
• Using USING HASH() which is invalid

3. Given two tables employees(emp_id, dept_id) and departments(dept_id, name), what join algorithm will PostgreSQL most likely use for this query?

EXPLAIN SELECT * FROM employees JOIN departments ON employees.dept_id = departments.dept_id;

Assuming both tables are large and departments.dept_id is indexed.

medium

A. Nested Loop Join

B. Merge Join

C. Cross Join

D. Hash Join

Solution

1. Step 1: Analyze table sizes and indexes

   Both tables are large, so Nested Loop is inefficient. Departments has an index on dept_id.

2. Step 2: Determine join algorithm choice

   Hash Join is preferred for large tables without sorted data. Merge Join requires sorted inputs, which is not guaranteed here.

3. Final Answer:

   Hash Join -> Option D

4. Quick Check:

   Large tables + no sorted data = Hash Join [OK]

Hint: Large tables with join keys use Hash Join by default [OK]

Common Mistakes:

• Assuming index forces Nested Loop Join
• Thinking Merge Join is automatic without sorting
• Confusing Cross Join with inner join

4. You wrote this query:

SELECT * FROM orders o JOIN customers c ON o.customer_id = c.customer_id;

But PostgreSQL is using a Nested Loop Join causing slow performance. Which fix will most likely improve performance by enabling a better join algorithm?

medium

A. Disable Nested Loop Join with SET enable_nestloop = off;

B. Create an index on orders.customer_id

C. Rewrite query using LEFT JOIN instead of JOIN

D. Add ORDER BY on customer_id in the query

Solution

1. Step 1: Identify why Nested Loop is slow

   Nested Loop is slow on large tables without indexes or when better joins exist but are not chosen.

2. Step 2: Force PostgreSQL to avoid Nested Loop

   Disabling Nested Loop join with SET enable_nestloop = off forces PostgreSQL to pick Hash or Merge Join, improving performance.

3. Final Answer:

   Disable Nested Loop Join with SET enable_nestloop = off; -> Option A

4. Quick Check:

   Disable Nested Loop to force better join [OK]

Hint: Disable nested loop join to force hash or merge join [OK]

Common Mistakes:

• Assuming adding index always fixes join choice
• Changing JOIN type without understanding join algorithms
• Adding ORDER BY does not affect join algorithm

5. You have two large sorted tables sales(date, product_id, amount) and products(product_id, name). You want to join them on product_id efficiently. Which join algorithm should you prefer and why?

hard

A. Merge Join, because it exploits sorted order for fast merging

B. Hash Join, because it hashes the smaller table for fast lookup

C. Nested Loop Join, because it works well with sorted data

D. Cross Join, because it combines all rows

Solution

1. Step 1: Identify join algorithm suited for sorted tables

   Merge Join is designed to efficiently join two sorted inputs by merging them in order.

2. Step 2: Compare with other join algorithms

   Nested Loop is inefficient for large tables, Hash Join ignores sorting, Cross Join produces Cartesian product.

3. Final Answer:

   Merge Join, because it exploits sorted order for fast merging -> Option A

4. Quick Check:

   Sorted tables + Merge Join = efficient join [OK]

Hint: Use Merge Join when both tables are sorted on join keys [OK]

Common Mistakes:

• Choosing Nested Loop for large sorted tables
• Ignoring sorting and picking Hash Join
• Confusing Cross Join with inner join