Index-only scans mental model in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the table using an index scan.

PostgreSQL

SELECT * FROM users WHERE [1] = 'active';

Drag options to blanks, or click blank then click option'

Astatus

Bid

Cname

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column not indexed in the WHERE clause.

Selecting columns not covered by the index.

The status column is used in the WHERE clause to filter rows. If an index exists on status, PostgreSQL can use an index scan to quickly find rows where status is 'active'.

2fill in blank

medium

Complete the code to create an index that supports index-only scans on the 'status' column.

PostgreSQL

CREATE INDEX idx_users_[1] ON users([2]);

Drag options to blanks, or click blank then click option'

Astatus

Bid

Cemail

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Creating an index on the wrong column.

Using a different column name in the index and query.

Creating an index on the status column allows PostgreSQL to perform index-only scans when queries filter by status.

3fill in blank

hard

Fix the error in the query to enable an index-only scan by selecting only indexed columns.

PostgreSQL

SELECT [1] FROM users WHERE status = 'active';

Drag options to blanks, or click blank then click option'

A*

Bemail

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting all columns with '*', which prevents index-only scans.

Selecting columns not covered by the index.

Index-only scans work when the query selects only columns present in the index. Selecting id (assuming it is indexed) allows the scan to avoid accessing the table.

4fill in blank

hard

Fill both blanks to create a covering index that supports index-only scans for queries filtering by 'status' and selecting 'email'.

PostgreSQL

CREATE INDEX idx_users_[1] ON users(status, [2]);

Drag options to blanks, or click blank then click option'

Astatus_email

Bstatus

Cemail

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Creating an index on only one column when two are needed.

Naming the index unrelated to its columns.

A multi-column index on status and email allows index-only scans for queries filtering by status and selecting email.

5fill in blank

hard

Fill all three blanks to write a query that uses an index-only scan by selecting only indexed columns and filtering by 'status'.

PostgreSQL

SELECT [1], [2] FROM users WHERE [3] = 'active';

Drag options to blanks, or click blank then click option'

Aid

Bemail

Cstatus

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting columns not in the index.

Filtering by a column not indexed.

Selecting id and email columns and filtering by status (all indexed) enables an index-only scan, avoiding table access.

Practice

1. What is the main advantage of an index-only scan in PostgreSQL?

easy

A. It reads data only from the index without accessing the main table.

B. It updates the index faster than a normal scan.

C. It locks the table to prevent concurrent writes.

D. It creates a new index automatically during query execution.

Solution

1. Step 1: Understand what an index-only scan does

   An index-only scan uses the index to get all needed data without reading the main table.

2. Step 2: Compare options to this behavior

   Only It reads data only from the index without accessing the main table. describes reading data solely from the index, which is the key benefit.

3. Final Answer:

   It reads data only from the index without accessing the main table. -> Option A

4. Quick Check:

   Index-only scan = reads from index only [OK]

Hint: Index-only scans avoid table reads by using index data only [OK]

Common Mistakes:

• Thinking index-only scans update data
• Assuming they lock tables
• Believing they create indexes automatically

2. Which of the following is a correct condition for PostgreSQL to use an index-only scan?

easy

A. The table has no indexes defined.

B. The index contains all columns needed by the query.

C. The query uses aggregate functions only.

D. The table is empty.

Solution

1. Step 1: Recall index-only scan requirements

   PostgreSQL can use index-only scans only if the index has all columns the query needs.

2. Step 2: Evaluate each option

   The index contains all columns needed by the query. matches the requirement; others do not enable index-only scans.

3. Final Answer:

   The index contains all columns needed by the query. -> Option B

4. Quick Check:

   Index-only scan requires full column coverage in index [OK]

Hint: Index-only scans need all query columns in the index [OK]

Common Mistakes:

• Thinking index-only scans work without indexes
• Assuming aggregates always use index-only scans
• Believing empty tables affect index-only scans

3. Given a table users(id, name, email) with an index on (id, email), which query can use an index-only scan?

medium

A. SELECT id, email FROM users WHERE id = 10;

B. SELECT name FROM users WHERE id = 10;

C. SELECT email FROM users WHERE name = 'Alice';

D. SELECT * FROM users WHERE email = '[e-mail]';

Solution

1. Step 1: Check index columns and query columns

   The index covers columns id and email. Query C requests only id and email.

2. Step 2: Determine if index-only scan is possible

   Query C can use index-only scan because all requested columns are in the index. Others request columns not in the index.

3. Final Answer:

   SELECT id, email FROM users WHERE id = 10; -> Option A

4. Quick Check:

   Query columns ⊆ index columns = index-only scan [OK]

Hint: Index-only scan if query columns are subset of index columns [OK]

Common Mistakes:

• Choosing queries requesting columns not in index
• Ignoring WHERE clause columns
• Assuming * always uses index-only scan

4. You have an index on (username, email) but your query SELECT email FROM users WHERE username = 'bob'; is not using an index-only scan. What could be the reason?

medium

A. The query uses a WHERE clause on username, so index-only scan is impossible.

B. The index does not include the email column.

C. The table's visibility map is not updated, so PostgreSQL must check the table.

D. PostgreSQL never uses index-only scans on text columns.

Solution

1. Step 1: Confirm index covers needed columns

   The index includes username and email, so columns are covered.

2. Step 2: Understand visibility map role

   Index-only scans require the visibility map to confirm tuples are visible without table access. If not updated, PostgreSQL reads the table.

3. Final Answer:

   The table's visibility map is not updated, so PostgreSQL must check the table. -> Option C

4. Quick Check:

   Visibility map must be updated for index-only scan [OK]

Hint: Visibility map must be updated for index-only scans [OK]

Common Mistakes:

• Assuming index missing columns
• Believing WHERE clause blocks index-only scan
• Thinking data type prevents index-only scans

5. You want to optimize a query SELECT id, status FROM orders WHERE status = 'shipped'; for index-only scans. The current index is on (id). What is the best way to enable index-only scans?

hard

A. Add a WHERE clause to filter only 'shipped' status.

B. Create a partial index on (id) where status = 'shipped'.

C. Drop the existing index and rely on sequential scan.

D. Create a new index on (status, id) including both columns.

Solution

1. Step 1: Identify columns needed for index-only scan

   The query selects id and status, so the index must cover both columns.

2. Step 2: Choose index that covers all columns

   Creating an index on (status, id) includes both columns, enabling index-only scans.

3. Final Answer:

   Create a new index on (status, id) including both columns. -> Option D

4. Quick Check:

   Index covering all query columns enables index-only scan [OK]

Hint: Index must cover all selected columns for index-only scan [OK]

Common Mistakes:

• Thinking partial index alone enables index-only scan
• Dropping index reduces performance
• Adding WHERE clause doesn't affect index structure