KEYS pattern matching (avoid in production) in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis KEYS command do?

The KEYS command finds all keys in the database that match a given pattern.

beginner

Why should you avoid using KEYS in production?

Because KEYS scans the entire database, it can slow down Redis and block other commands, especially with many keys.

intermediate

What is a safer alternative to KEYS for pattern matching in Redis?

Use the SCAN command, which iterates over keys incrementally without blocking Redis.

beginner

Give an example pattern you might use with KEYS.

For example, KEYS user:* finds all keys starting with user:.

intermediate

What kind of patterns can KEYS match?

It supports glob-style patterns like * (any characters), ? (single character), and [abc] (character sets).

What does the Redis KEYS command return?

A All keys matching a pattern

B The value of a key

C The number of keys in the database

D The type of a key

KEYS returns all keys matching the given pattern.

Why is KEYS not recommended in production?

A It deletes keys

B It blocks Redis and slows performance

C It only works on small databases

D It returns incorrect results

KEYS scans all keys at once, which can block Redis and slow down performance.

Which command is better for safely scanning keys in Redis?

A SCAN

B SET

C GET

D DEL

SCAN iterates keys incrementally without blocking Redis.

What pattern would find all keys starting with 'session:'?

A session[]

B session?

C *session

D session:*

The pattern session:* matches keys starting with 'session:'.

Which wildcard matches exactly one character in Redis patterns?

A *

B #

C ?

D [abc]

The ? wildcard matches exactly one character.

Explain what the Redis KEYS command does and why it should be avoided in production.

Think about how scanning all keys affects Redis performance.

Describe the difference between KEYS and SCAN commands in Redis.

Consider how each command handles large numbers of keys.

Practice

1. What does the Redis command KEYS * do?

easy

A. Returns all keys in the current database

B. Deletes all keys in the current database

C. Returns the number of keys in the current database

D. Sets a new key with a wildcard name

Solution

1. Step 1: Understand the KEYS command

   The KEYS command searches for keys matching a given pattern.

2. Step 2: Analyze the pattern *

   The pattern * matches all keys in the current database.

3. Final Answer:

   Returns all keys in the current database -> Option A

4. Quick Check:

   KEYS * lists all keys [OK]

Hint: Remember * matches everything in KEYS command [OK]

Common Mistakes:

• Thinking KEYS deletes keys
• Confusing KEYS with COUNT commands
• Assuming KEYS sets keys

2. Which of the following is the correct unquoted syntax to find keys starting with 'user:' using KEYS?

easy

A. KEYS user:*

B. KEYS user*

C. KEYS 'user:*'

D. KEYS 'user*'

Solution

1. Step 1: Understand pattern syntax in KEYS

   Patterns do not require quotes; wildcards like * match any characters.

2. Step 2: Match keys starting with 'user:'

   The pattern should be user:* without quotes to match keys starting with 'user:'.

3. Final Answer:

   KEYS user:* -> Option A

4. Quick Check:

   Use unquoted patterns with * wildcard [OK]

Hint: Use unquoted patterns with * for KEYS [OK]

Common Mistakes:

• Adding quotes around pattern
• Using colon without wildcard
• Using wrong wildcard placement

3. Given keys: user:1, user:2, admin:1, what is the output of KEYS user:*?

medium

A. ["admin:1"]

B. ["user:1", "user:2", "admin:1"]

C. ["user:1", "user:2"]

D. []

Solution

1. Step 1: Identify keys matching pattern user:*

   Keys starting with 'user:' are user:1 and user:2.

2. Step 2: Exclude keys not matching pattern

   admin:1 does not start with 'user:', so it is excluded.

3. Final Answer:

   ["user:1", "user:2"] -> Option C

4. Quick Check:

   Pattern matches keys starting with 'user:' [OK]

Hint: Match only keys starting with pattern prefix [OK]

Common Mistakes:

• Including keys with different prefixes
• Returning empty list incorrectly
• Confusing pattern syntax

4. What is wrong with this command: KEYS user?*?

medium

A. The question mark is not a valid wildcard in KEYS

B. The pattern should be quoted

C. The command will delete keys instead of listing

D. The pattern matches only keys with exactly one character after 'user' followed by any characters

Solution

1. Step 1: Understand the question mark wildcard in KEYS

   The question mark ? matches exactly one character in Redis patterns.

2. Step 2: Analyze the pattern user?*

   This pattern matches keys starting with 'user', followed by exactly one character, then any characters.

3. Final Answer:

   The pattern matches only keys with exactly one character after 'user' -> Option D

4. Quick Check:

   ? matches exactly one character [OK]

Hint: Remember ? matches exactly one character in KEYS [OK]

Common Mistakes:

• Thinking ? is invalid wildcard
• Assuming pattern deletes keys
• Forgetting ? matches exactly one character

5. You want to find all keys containing the word 'session' anywhere in their name without blocking Redis in production. Which approach is best?

hard

A. Use KEYS *session* command directly

B. Use SCAN 0 MATCH *session* iteratively

C. Use GET *session* command

D. Use DEL *session* to remove keys first

Solution

1. Step 1: Understand why KEYS is discouraged in production

   KEYS blocks Redis and can cause performance issues on large datasets.

2. Step 2: Use SCAN with MATCH for safe iteration

   SCAN iterates keys incrementally without blocking, using MATCH *session* to filter keys containing 'session'.

3. Final Answer:

   Use SCAN 0 MATCH *session* iteratively -> Option B

4. Quick Check:

   SCAN is safe for production key matching [OK]

Hint: Use SCAN with MATCH to avoid blocking Redis [OK]

Common Mistakes:

• Using KEYS in production causing blocking
• Using GET with wildcards (invalid)
• Deleting keys without checking