Expression indexes in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an expression index on the lower case of the column 'name'.

PostgreSQL

CREATE INDEX idx_lower_name ON users ([1]);

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

Aname

BUPPER(name)

CLOWER(name)

Dlength(name)

Attempts:

3 left

💡 Hint

Common Mistakes

Using the column name directly without the LOWER function.

Using UPPER instead of LOWER.

The expression index uses LOWER(name) to index the lowercase version of the 'name' column.

2fill in blank

medium

Complete the code to create an expression index on the first 10 characters of the 'email' column.

PostgreSQL

CREATE INDEX idx_email_prefix ON users ([1]);

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

ASUBSTRING(email FROM 1 FOR 10)

BLEFT(email, 5)

CRIGHT(email, 10)

DLENGTH(email)

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT(email, 5) which only gets 5 characters.

Using RIGHT(email, 10) which gets the last 10 characters.

The expression index uses SUBSTRING(email FROM 1 FOR 10) to index the first 10 characters of the 'email' column.

3fill in blank

hard

Fix the error in the expression index creation by completing the code correctly.

PostgreSQL

CREATE INDEX idx_trim_name ON users ([1]);

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

Aname TRIM()

BTRIM name

CTRIM(name, ' ')

DTRIM(name)

Attempts:

3 left

💡 Hint

Common Mistakes

Writing TRIM without parentheses.

Trying to pass two arguments to TRIM.

The correct syntax for trimming spaces is TRIM(name). Options A, B, and D are invalid syntax. Option D is invalid because TRIM in PostgreSQL does not take two arguments like that.

4fill in blank

hard

Fill both blanks to create an expression index on the lower case of the first 5 characters of 'username'.

PostgreSQL

CREATE INDEX idx_lower_prefix ON users ([1]([2](username, 1, 5)));

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

ALOWER

BSUBSTRING

Cusername

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using email instead of username.

Reversing the order of functions.

The expression index applies LOWER to the SUBSTRING of 'username'. The correct order is LOWER(SUBSTRING(username, 1, 5)).

5fill in blank

hard

Fill all three blanks to create an expression index on the trimmed, lowercased 'city' column.

PostgreSQL

CREATE INDEX idx_trim_lower_city ON locations ([1]([2]([3])));

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

ALOWER

BTRIM

Ccity

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'state' instead of 'city'.

Reversing the order of LOWER and TRIM.

The expression index applies LOWER to TRIM applied to the 'city' column: LOWER(TRIM(city)).

Practice

1. What is the main purpose of an expression index in PostgreSQL?

easy

A. To create a backup of the database

B. To store data in a compressed format

C. To speed up queries that filter or sort by a calculated expression

D. To enforce foreign key constraints

Solution

1. Step 1: Understand what expression indexes do

   Expression indexes are special indexes built on the result of an expression or function, not just a column.

2. Step 2: Identify their main use

   They help speed up queries that filter or sort using that expression, improving performance.

3. Final Answer:

   To speed up queries that filter or sort by a calculated expression -> Option C

4. Quick Check:

   Expression index purpose = speed up expression queries [OK]

Hint: Expression indexes speed up queries using expressions [OK]

Common Mistakes:

• Confusing expression indexes with data compression
• Thinking expression indexes create backups
• Mixing expression indexes with constraints

2. Which of the following is the correct syntax to create an expression index on the lowercased username column in PostgreSQL?

easy

A. CREATE INDEX idx_lower_username ON users (LOWER(username));

B. CREATE INDEX idx_lower_username ON users ((LOWER(username)));

C. CREATE INDEX idx_lower_username ON users [LOWER(username)];

D. CREATE INDEX idx_lower_username ON users {LOWER(username)};

Solution

1. Step 1: Recall expression index syntax

   Expression indexes require double parentheses around the expression inside the index definition.

2. Step 2: Check each option

   CREATE INDEX idx_lower_username ON users ((LOWER(username))); uses double parentheses correctly: ((LOWER(username))). Options B, C, and D use incorrect syntax.

3. Final Answer:

   CREATE INDEX idx_lower_username ON users ((LOWER(username))); -> Option B

4. Quick Check:

   Expression index syntax = double parentheses [OK]

Hint: Use double parentheses for expressions in CREATE INDEX [OK]

Common Mistakes:

• Using single parentheses instead of double
• Using square or curly brackets
• Missing parentheses around the expression

3. Given the table products(id INT, price NUMERIC) and the index:
CREATE INDEX idx_discounted_price ON products ((price * 0.9));
What will the query below use to speed up filtering?
SELECT * FROM products WHERE price * 0.9 < 100;

medium

A. It will perform a full table scan ignoring the index

B. It will use a default index on price if it exists

C. It will cause a syntax error due to the expression

D. It will use the expression index on (price * 0.9) to speed up the query

Solution

1. Step 1: Understand the expression index usage

   The index is created on the expression (price * 0.9), matching the WHERE clause expression exactly.

2. Step 2: Match query filter with index expression

   Since the query filters on price * 0.9 < 100, PostgreSQL can use the expression index to speed up filtering.

3. Final Answer:

   It will use the expression index on (price * 0.9) to speed up the query -> Option D

4. Quick Check:

   Matching expression in WHERE = index used [OK]

Hint: Expression index used if query expression matches exactly [OK]

Common Mistakes:

• Assuming default indexes are used instead
• Thinking expression indexes cause errors
• Believing full table scan always happens

4. You tried to create an expression index with:
CREATE INDEX idx_expr ON sales (price * discount);
But PostgreSQL returned a syntax error. What is the problem?

medium

A. Expression indexes require double parentheses around the expression

B. You cannot create indexes on expressions involving multiplication

C. The table name is missing

D. The index name is invalid

Solution

1. Step 1: Check expression index syntax

   Expression indexes must have the expression enclosed in double parentheses to be valid.

2. Step 2: Identify the syntax error cause

   The given statement uses single parentheses, causing a syntax error.

3. Final Answer:

   Expression indexes require double parentheses around the expression -> Option A

4. Quick Check:

   Double parentheses fix syntax error [OK]

Hint: Use double parentheses for expressions to avoid syntax errors [OK]

Common Mistakes:

• Using single parentheses for expressions
• Thinking multiplication is not allowed
• Ignoring syntax error details

5. You want to speed up queries filtering by the first 3 letters of a city column in a locations table. Which expression index will best help?
CREATE INDEX idx_city_prefix ON locations (???);

hard

A. LEFT(city, 3)

B. SUBSTRING(city FROM 1 FOR 3)

C. city[1:3]

D. city LIKE '___%'

Solution

1. Step 1: Identify correct expression syntax for substring

   PostgreSQL supports the function LEFT(string, n) to get the first n characters.

2. Step 2: Evaluate options for expression index

   LEFT(city, 3) uses LEFT(city, 3) correctly inside the index. SUBSTRING(city FROM 1 FOR 3) uses SUBSTRING but syntax is less common and may be less efficient. city[1:3] is invalid syntax. city LIKE '___%' is a condition, not an expression.

3. Final Answer:

   LEFT(city, 3) -> Option A

4. Quick Check:

   LEFT function best for prefix expression index [OK]

Hint: Use LEFT(column, n) for prefix expression indexes [OK]

Common Mistakes:

• Using invalid substring syntax
• Confusing LIKE pattern with expression
• Using array slice syntax on strings