Sequential scan vs index scan in PostgreSQL

Introduction

We want to find data quickly in a database. Sequential scan and index scan are two ways to look for data.

When you want to read all rows in a table.

When you want to find a few rows using a search condition.

When the table is small and reading all rows is fast.

When the table is large but you have an index on the searched column.

When you want to understand how PostgreSQL finds data.

Syntax

PostgreSQL

EXPLAIN SELECT * FROM table_name WHERE column_name = value;

This command shows how PostgreSQL plans to find data.

It can show if it uses a sequential scan or an index scan.

Examples

Shows how PostgreSQL searches for employee with id 5.

PostgreSQL

EXPLAIN SELECT * FROM employees WHERE id = 5;

Shows how PostgreSQL reads all employees (likely sequential scan).

PostgreSQL

EXPLAIN SELECT * FROM employees;

Creates an index on the name column to speed up searches by name.

PostgreSQL

CREATE INDEX idx_name ON employees(name);

Shows if PostgreSQL uses the index on name to find Alice.

PostgreSQL

EXPLAIN SELECT * FROM employees WHERE name = 'Alice';

Sample Program

This creates a small employees table, adds three rows, and shows how PostgreSQL finds the employee with id 2.

PostgreSQL

CREATE TABLE employees (id SERIAL PRIMARY KEY, name TEXT, age INT);
INSERT INTO employees (name, age) VALUES ('Alice', 30), ('Bob', 25), ('Carol', 27);
EXPLAIN SELECT * FROM employees WHERE id = 2;

Important Notes

A sequential scan reads every row one by one. It is simple but can be slow on big tables.

An index scan uses a special data structure to jump directly to matching rows. It is faster for selective queries.

PostgreSQL decides automatically which scan to use based on table size and query.

Summary

Sequential scan reads all rows; good for small tables or full reads.

Index scan uses an index to find rows faster; good for selective queries.

Use EXPLAIN to see which scan PostgreSQL chooses.

Practice

1. What does a sequential scan do in PostgreSQL?

easy

A. Reads only the first 10 rows of a table

B. Uses an index to find specific rows quickly

C. Deletes rows based on a condition

D. Reads every row in the table one by one

Solution

1. Step 1: Understand sequential scan behavior

   A sequential scan reads all rows in the table from start to end without using any index.

2. Step 2: Compare with other options

   Using an index is an index scan, reading only first 10 rows or deleting rows are unrelated actions.

3. Final Answer:

   Reads every row in the table one by one -> Option D

4. Quick Check:

   Sequential scan = full table read [OK]

Hint: Sequential scan reads all rows, no index used [OK]

Common Mistakes:

• Confusing sequential scan with index scan
• Thinking sequential scan reads only some rows
• Assuming sequential scan deletes rows

2. Which of the following is the correct way to see if PostgreSQL uses an index scan or sequential scan?

easy

A. SELECT * FROM table WHERE id = 1;

B. EXPLAIN SELECT * FROM table WHERE id = 1;

C. CREATE INDEX ON table(id);

D. DROP INDEX index_name;

Solution

1. Step 1: Identify command to check query plan

   The EXPLAIN command shows how PostgreSQL executes a query, including scan type.

2. Step 2: Eliminate other options

   SELECT runs query but doesn't show plan; CREATE and DROP INDEX modify indexes, not show plans.

3. Final Answer:

   EXPLAIN SELECT * FROM table WHERE id = 1; -> Option B

4. Quick Check:

   EXPLAIN shows scan type [OK]

Hint: Use EXPLAIN before query to see scan type [OK]

Common Mistakes:

• Running SELECT without EXPLAIN to check scan
• Confusing index creation with scan checking
• Using DROP INDEX to check scans

3. Given a table users(id SERIAL PRIMARY KEY, name TEXT) with 1 million rows, which scan is PostgreSQL likely to use for this query?
SELECT * FROM users WHERE id = 500000;

medium

A. Index scan using the primary key index

B. Sequential scan scanning all 1 million rows

C. Bitmap heap scan reading random rows

D. No scan, query will fail

Solution

1. Step 1: Analyze query condition and table size

   The query filters by primary key id, which has an index, and the table is large (1 million rows).

2. Step 2: Determine efficient scan type

   PostgreSQL uses an index scan to quickly find the single matching row instead of scanning all rows.

3. Final Answer:

   Index scan using the primary key index -> Option A

4. Quick Check:

   Selective query on indexed column = index scan [OK]

Hint: Selective query on indexed column uses index scan [OK]

Common Mistakes:

• Assuming sequential scan for large tables with indexed filter
• Confusing bitmap heap scan with index scan
• Thinking query fails without reason

4. You wrote this query:
SELECT * FROM orders WHERE customer_id = 123;
But EXPLAIN shows a sequential scan instead of an index scan. What could be the reason?

medium

A. There is no index on customer_id

B. The table is empty

C. The query syntax is incorrect

D. PostgreSQL always uses sequential scan

Solution

1. Step 1: Check index presence on filter column

   If no index exists on customer_id, PostgreSQL must scan all rows sequentially.

2. Step 2: Evaluate other options

   Empty table would still show scan but no rows; syntax is correct; PostgreSQL chooses scan type based on indexes.

3. Final Answer:

   There is no index on customer_id -> Option A

4. Quick Check:

   No index on filter column = sequential scan [OK]

Hint: No index on filter column causes sequential scan [OK]

Common Mistakes:

• Assuming syntax error causes scan type
• Thinking PostgreSQL always uses sequential scan
• Ignoring missing index as cause

5. You have a large table products with millions of rows and an index on category_id. You run:
SELECT * FROM products WHERE category_id IN (1, 2, 3, 4, 5);
PostgreSQL chooses a sequential scan instead of an index scan. Why might this happen?

hard

A. The index on category_id is corrupted

B. Sequential scan is always faster for any query

C. The query is not selective enough; many rows match

D. The IN clause is invalid syntax

Solution

1. Step 1: Understand query selectivity

   The query matches many rows because it filters on multiple categories, reducing selectivity.

2. Step 2: Explain PostgreSQL scan choice

   When many rows match, PostgreSQL prefers sequential scan as it can be faster than many index lookups.

3. Final Answer:

   The query is not selective enough; many rows match -> Option C

4. Quick Check:

   Low selectivity = sequential scan preferred [OK]

Hint: Low selectivity queries often use sequential scan [OK]

Common Mistakes:

• Assuming index corruption without evidence
• Thinking sequential scan is always slower
• Believing IN clause is invalid syntax