Sequential scan vs index scan in PostgreSQL - Quick Revision & Key Differences

Recall & Review

beginner

What is a sequential scan in PostgreSQL?

A sequential scan reads every row in a table one by one to find matching rows. It is simple but can be slow for large tables.

beginner

What is an index scan in PostgreSQL?

An index scan uses an index to quickly find rows that match a condition, without reading the whole table. It is faster for selective queries.

intermediate

When does PostgreSQL prefer a sequential scan over an index scan?

PostgreSQL prefers a sequential scan when a large portion of the table needs to be read, because reading the whole table can be faster than using the index.

beginner

How does an index scan improve query performance?

An index scan improves performance by quickly locating only the rows that match the query condition, reducing the number of rows read from the table.

intermediate

What is a downside of using an index scan?

Index scans can be slower if the query matches many rows because it may require many random reads, which are slower than reading sequentially.

What does a sequential scan do in PostgreSQL?

A Uses an index to find rows quickly

B Reads every row in the table one by one

C Only reads the first 10 rows

D Reads rows randomly

A sequential scan reads every row in the table one by one to find matching rows.

When is an index scan usually faster than a sequential scan?

A When the query matches very few rows

B When the table has no indexes

C When the table is empty

D When the query matches many rows

Index scans are faster when the query matches very few rows because it can quickly locate them using the index.

Why might PostgreSQL choose a sequential scan over an index scan?

A Because the query is very simple

B Because the table is very small

C Because the index is corrupted

D Because the query matches a large portion of the table

PostgreSQL chooses a sequential scan when the query matches a large portion of the table, making it faster to read all rows sequentially.

What is a disadvantage of index scans?

A They can be slower if many rows match the query

B They always read the whole table

C They do not use indexes

D They only work on small tables

Index scans can be slower if many rows match because they may cause many random disk reads.

Which scan type reads rows in the order they are stored on disk?

A Index scan

B Bitmap scan

C Sequential scan

D Hash scan

Sequential scans read rows in the order they are stored on disk, one after another.

Explain the difference between a sequential scan and an index scan in PostgreSQL.

Think about how each method reads data and when each is faster.

Describe when PostgreSQL might choose a sequential scan instead of an index scan.

Consider the cost of reading many rows randomly vs sequentially.

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