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PostgreSQL Performance & Query Optimization · Lesson

Covering Indexes and Index-Only Scans

Discover how covering indexes can eliminate table access and enable highly efficient index-only scans.

Covering Indexes and Index-Only Scans is a free PostgreSQL Performance & Query Optimization lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the PostgreSQL Performance & Query Optimization learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What are Covering Indexes?

Welcome to our lesson on Covering Indexes and Index-Only Scans! These advanced indexing techniques can dramatically boost your PostgreSQL query performance.

A regular index helps PostgreSQL quickly find rows based on certain column values. Think of it like a book's index: it tells you which page to go to for a topic.

The Cost of Table Access

When you use a standard index, PostgreSQL first finds the row's location (its tuple ID or CTID) in the index. Then, it has to go to the actual table to fetch the rest of the data for that row.

This extra step, called a table lookup or heap fetch, can be slow, especially if many rows are involved or if the table data isn't in memory.

Indexing All You Need

A covering index is an index that contains all the columns needed to fulfill a query, not just the columns in the WHERE clause. This means PostgreSQL can answer the query by reading only the index, without ever touching the main table data.

It 'covers' the query's data requirements entirely.

Building a Covering Index

To create a covering index, you include all the columns your query might select or filter on. PostgreSQL supports including non-key columns using the INCLUDE clause, which stores them without making them part of the primary sort key.

Let's create a table and then a covering index.

CREATE TABLE products (
  product_id SERIAL PRIMARY KEY,
  name VARCHAR(100) NOT NULL,
  price DECIMAL(10, 2) NOT NULL,
  category VARCHAR(50)
);

INSERT INTO products (name, price, category) VALUES
('Laptop', 1200.00, 'Electronics'),
('Keyboard', 75.00, 'Accessories'),
('Mouse', 25.00, 'Accessories'),
('Monitor', 300.00, 'Electronics');

-- Covering index for queries on category that also select name and price
CREATE INDEX idx_products_category_cover_name_price
ON products (category) INCLUDE (name, price);

Introducing Index-Only Scans

When a query can be fully satisfied by a covering index, PostgreSQL performs an Index-Only Scan. This is a highly efficient operation because the database engine doesn't need to read any data blocks from the main table (the 'heap').

It's like finding all the information you need directly in the book's index, without ever turning to the main chapters.

MVCC & Index-Only Scans

PostgreSQL uses MVCC (Multi-Version Concurrency Control). For an Index-Only Scan to work, PostgreSQL must ensure that the versions of the rows it finds in the index are visible to the current transaction.

It does this by checking the table's visibility map, a special data structure that tracks which table pages contain only 'all-visible' tuples. If a page is marked all-visible, no heap fetch is needed to check row visibility.

Confirming with EXPLAIN

You can verify if PostgreSQL is using an Index-Only Scan by checking the query plan with EXPLAIN ANALYZE. Look for Index Only Scan in the output.

Let's try a query that our covering index should handle.

EXPLAIN ANALYZE
SELECT name, price
FROM products
WHERE category = 'Electronics';

Benefits: Speed & Efficiency

Index-Only Scans offer several key advantages:

  • Reduced Disk I/O: Fewer disk reads are needed because the main table is skipped.
  • Faster Query Execution: Less I/O directly translates to quicker query responses.
  • Better Cache Utilization: Indexes are often smaller and more frequently accessed, leading to better use of memory caches.
  • Concurrency: Less contention on table data blocks.

Trade-offs & Considerations

While powerful, covering indexes aren't a silver bullet:

  • Larger Indexes: Including more columns makes the index physically larger, consuming more disk space.
  • Slower Writes: Updates, inserts, and deletes to the main table also require updating the index, which can be slower for larger indexes.
  • Maintenance: Larger indexes might take longer to VACUUM.

Use them strategically for read-heavy queries that frequently access a specific subset of columns.

Practical Covering Index Example

Consider a `users` table where you often query user names and emails based on their registration date. An index on `registration_date` including `name` and `email` would be a perfect candidate for a covering index.

CREATE TABLE users (
  user_id SERIAL PRIMARY KEY,
  username VARCHAR(50) NOT NULL,
  email VARCHAR(100) NOT NULL,
  registration_date DATE NOT NULL
);

INSERT INTO users (username, email, registration_date) VALUES
('alice', 'alice@example.com', '2023-01-15'),
('bob', 'bob@example.com', '2023-02-20'),
('charlie', 'charlie@example.com', '2023-01-25');

CREATE INDEX idx_users_regdate_cover_name_email
ON users (registration_date) INCLUDE (username, email);

EXPLAIN ANALYZE
SELECT username, email
FROM users
WHERE registration_date BETWEEN '2023-01-01' AND '2023-01-31';

Quick Check: Index Scans

Based on what you've learned, identify the correct statements about Index-Only Scans in PostgreSQL.

Recap: Covering Indexes

You've learned that covering indexes include all columns needed by a query, enabling highly efficient Index-Only Scans. These scans bypass the main table, significantly reducing disk I/O and speeding up read-heavy queries.

Remember to use EXPLAIN ANALYZE to confirm their usage and consider the trade-offs of increased index size and potential write overhead. Happy optimizing!

Frequently asked questions

Is the “Covering Indexes and Index-Only Scans” lesson free?

Yes — the full text of “Covering Indexes and Index-Only Scans” is free to read here on the web, and the PostgreSQL Performance & Query Optimization course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the PostgreSQL Performance & Query Optimization course, upgrade to CoddyKit PRO.

What will I learn in “Covering Indexes and Index-Only Scans”?

Discover how covering indexes can eliminate table access and enable highly efficient index-only scans. You practise PostgreSQL Performance & Query Optimization with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start PostgreSQL Performance & Query Optimization?

No prior experience is required. PostgreSQL Performance & Query Optimization on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Covering Indexes and Index-Only Scans” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this PostgreSQL Performance & Query Optimization lesson?

Yes. Every PostgreSQL Performance & Query Optimization lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Hash, GIN, and GiST Indexes
  2. Partial and Expression Indexes
  3. Covering Indexes and Index-Only Scans
  4. BRIN Indexes for Large Sequential Data
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