Database indexes in Supabase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a database index?

A database index is like a shortcut in a book's table of contents. It helps the database find data faster without looking at every row.

beginner

How does an index improve database queries?

An index speeds up queries by letting the database quickly locate the data instead of scanning the whole table.

intermediate

What is a downside of using too many indexes?

Too many indexes slow down data changes like inserts, updates, and deletes because the indexes must be updated too.

beginner

What types of columns are good candidates for indexing?

Columns used often in search conditions or joins, like IDs or foreign keys, are good for indexes.

intermediate

What is a unique index?

A unique index ensures that all values in a column are different, preventing duplicate entries.

What does a database index do?

A Deletes duplicate data

B Encrypts the database

C Speeds up data retrieval

D Backs up data automatically

A database index helps the database find data faster, improving query speed.

Which column is best to index?

A A column used in search queries

B A column with mostly unique values

C A column rarely used in queries

D A column with many NULL values

Columns used often in search queries benefit most from indexing.

What is a risk of having too many indexes?

A Faster queries

B Slower data updates

C More storage space saved

D Automatic data backup

Too many indexes slow down inserts, updates, and deletes because indexes must be updated.

What does a unique index enforce?

A Automatic data replication

B Faster data deletion

C Encrypted data storage

D No duplicate values in a column

A unique index ensures all values in the indexed column are different.

When should you avoid adding an index?

A On columns rarely used in queries

B On columns used in joins

C On primary key columns

D On foreign key columns

Indexing columns rarely used in queries wastes resources and slows data changes.

Explain what a database index is and how it helps with query performance.

Think of a book's table of contents.

Describe the trade-offs of using indexes in a database.

Indexes help reading but can slow writing.

Practice

1. What is the main purpose of creating a database index in Supabase?

easy

A. To speed up data searches and queries

B. To store more data in the database

C. To backup the database automatically

D. To encrypt the data for security

Solution

1. Step 1: Understand what an index does

   An index creates a quick lookup structure to find data faster.

2. Step 2: Compare options with index purpose

   Only speeding up searches matches the index function.

3. Final Answer:

   To speed up data searches and queries -> Option A

4. Quick Check:

   Index = Faster search [OK]

Hint: Indexes speed up searches, not storage or security [OK]

Common Mistakes:

• Thinking indexes store more data
• Confusing indexes with backups
• Assuming indexes encrypt data

2. Which of the following is the correct SQL syntax to create an index on the column email in a Supabase table named users?

easy

A. CREATE INDEX email ON users;

B. INDEX CREATE users.email;

C. CREATE INDEX ON users (email);

D. CREATE users INDEX email;

Solution

1. Step 1: Recall correct SQL index syntax

   The standard syntax is: CREATE INDEX index_name ON table_name (column_name); but index_name is optional.

2. Step 2: Match syntax with options

   CREATE INDEX ON users (email); uses correct syntax without specifying index name, which is allowed.

3. Final Answer:

   CREATE INDEX ON users (email); -> Option C

4. Quick Check:

   CREATE INDEX ON table (column) [OK]

Hint: Use CREATE INDEX ON table (column) for indexes [OK]

Common Mistakes:

• Placing column before table incorrectly
• Using wrong keyword order
• Omitting ON keyword

3. Given the table orders with an index on customer_id, what will be the effect of this query?

SELECT * FROM orders WHERE customer_id = 123;

medium

A. The query will return no results because indexes filter data

B. The query will ignore the index and scan the whole table

C. The query will fail due to missing index

D. The query will run faster by using the index on customer_id

Solution

1. Step 1: Understand index usage in queries

   Indexes help the database find matching rows quickly when filtering by indexed columns.

2. Step 2: Analyze the query filter

   The query filters by customer_id, which has an index, so the index will be used.

3. Final Answer:

   The query will run faster by using the index on customer_id -> Option D

4. Quick Check:

   Indexed column filter = faster query [OK]

Hint: Queries filtering indexed columns run faster [OK]

Common Mistakes:

• Thinking index causes query failure
• Assuming index filters out data
• Believing index is ignored automatically

4. You created multiple indexes on a Supabase table, but now data insertions are very slow. What is the most likely cause?

medium

A. Too many indexes slow down data changes

B. Indexes cause data loss during insert

C. Indexes prevent data from being inserted

D. Indexes automatically delete old data

Solution

1. Step 1: Recall how indexes affect data changes

   Each index must be updated when data changes, so many indexes slow inserts and updates.

2. Step 2: Match symptoms with cause

   Slow insertions match the effect of too many indexes.

3. Final Answer:

   Too many indexes slow down data changes -> Option A

4. Quick Check:

   Many indexes = slower inserts [OK]

Hint: More indexes = slower writes, fewer indexes = faster writes [OK]

Common Mistakes:

• Thinking indexes cause data loss
• Assuming indexes block inserts
• Believing indexes delete data automatically

5. You want to optimize a Supabase table products that is frequently searched by category and price. Which index strategy is best to improve query speed without slowing inserts too much?

hard

A. Create no indexes to keep inserts fast

B. Create a composite index on (category, price)

C. Create an index only on price

D. Create separate indexes on category and price

Solution

1. Step 1: Understand composite vs separate indexes

   A composite index on (category, price) helps queries filtering both columns efficiently.

2. Step 2: Consider insert performance

   One composite index is faster to update than two separate indexes, balancing speed and insert cost.

3. Step 3: Evaluate options

   Create a composite index on (category, price) optimizes queries on both columns with minimal insert slowdown.

4. Final Answer:

   Create a composite index on (category, price) -> Option B

5. Quick Check:

   Composite index = faster multi-column queries [OK]

Hint: Use composite indexes for multi-column searches [OK]

Common Mistakes:

• Creating too many separate indexes
• Indexing only one column when both are searched
• Avoiding indexes and hurting query speed