Bird
Raised Fist0
Redisquery~5 mins

ZRANGEBYSCORE for score-based queries in Redis - Time & Space Complexity

Choose your learning style10 modes available
LearnWhyDeepVisualTryChallengeProjectRecallTime
Time Complexity: ZRANGEBYSCORE for score-based queries
O(log n + m)
Understanding Time Complexity

When we use ZRANGEBYSCORE in Redis, we want to find items within a score range quickly.

We ask: How does the time to get these items grow as the data grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.


# Get all members with scores between 50 and 100
ZRANGEBYSCORE mysortedset 50 100

# Optionally limit results
ZRANGEBYSCORE mysortedset 50 100 LIMIT 0 10

This code fetches members from a sorted set whose scores fall between 50 and 100, optionally limiting the number of results.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

  • Primary operation: Scanning the sorted set to find members in the score range.
  • How many times: Depends on how many members fall within the score range; Redis uses a skiplist to jump quickly.
How Execution Grows With Input

As the sorted set grows, finding the start point is fast, but returning more results takes more time.

Input Size (n)Approx. Operations
10About 3 to 5 steps to find start, plus up to 10 to return
100About 5 to 7 steps to find start, plus up to 100 to return
1000About 7 to 10 steps to find start, plus up to 1000 to return

Pattern observation: Finding where to start grows slowly, but returning more results grows linearly with how many are returned.

Final Time Complexity

Time Complexity: O(log n + m)

This means it takes a small amount of time to find where to start, plus time proportional to how many results you get back.

Common Mistake

[X] Wrong: "ZRANGEBYSCORE always takes the same time no matter how many results it returns."

[OK] Correct: The time to find the start is fast, but returning many results takes longer because Redis must send each member back.

Interview Connect

Understanding how Redis handles score-based queries helps you explain efficient data retrieval in real systems.

Self-Check

"What if we add a LIMIT clause to always return only 5 results? How would the time complexity change?"

Practice

(1/5)
1. What does the Redis command ZRANGEBYSCORE do?
easy
A. It returns the rank of a member in a sorted set.
B. It deletes members from a sorted set by score.
C. It adds a member with a score to a sorted set.
D. It returns members in a sorted set within a given score range.

Solution

  1. Step 1: Understand the command purpose

    ZRANGEBYSCORE is used to get members from a sorted set based on their scores.

  2. Step 2: Compare options with command behavior

    Only It returns members in a sorted set within a given score range. correctly describes returning members within a score range.

  3. Final Answer:

    It returns members in a sorted set within a given score range. -> Option D

  4. Quick Check:

    ZRANGEBYSCORE = returns members by score range [OK]
Hint: ZRANGEBYSCORE fetches by score range, not by rank or deletion [OK]
Common Mistakes:
  • Confusing ZRANGEBYSCORE with ZREM (deletion)
  • Thinking it returns rank instead of members
  • Mixing it up with ZADD (adding members)
2. Which of the following is the correct syntax to get members with scores between 10 and 20 from sorted set myzset including their scores?
easy
A. ZRANGEBYSCORE myzset 10 20 INCLSCORES
B. ZRANGEBYSCORE myzset 10 20 WITHSCORE
C. ZRANGEBYSCORE myzset 10 20 WITHSCORES
D. ZRANGEBYSCORE myzset 10 20 SCORES

Solution

  1. Step 1: Recall correct option for including scores

    The correct keyword to include scores in output is WITHSCORES.

  2. Step 2: Check syntax correctness

    ZRANGEBYSCORE myzset 10 20 WITHSCORES uses the exact correct syntax: ZRANGEBYSCORE myzset 10 20 WITHSCORES.

  3. Final Answer:

    ZRANGEBYSCORE myzset 10 20 WITHSCORES -> Option C

  4. Quick Check:

    WITHSCORES keyword includes scores [OK]
Hint: Use WITHSCORES exactly to get scores with members [OK]
Common Mistakes:
  • Misspelling WITHSCORES
  • Using WITHSCORE or SCORES instead
  • Omitting WITHSCORES when scores are needed
3. Given the sorted set myzset with members and scores:
"a"=5, "b"=10, "c"=15, "d"=20, "e"=25
What is the result of ZRANGEBYSCORE myzset 10 20 LIMIT 1 2?
medium
A. ["c", "d"]
B. ["b", "c", "d"]
C. ["b", "c"]
D. ["a", "b"]

Solution

  1. Step 1: Identify members in score range 10 to 20

    Members with scores 10, 15, 20 are "b", "c", "d".

  2. Step 2: Apply LIMIT 1 2

    LIMIT 1 2 means skip 1 member, then return next 2 members. Skipping "b", next two are "c", "d".

  3. Final Answer:

    ["c", "d"] -> Option A

  4. Quick Check:

    LIMIT skips then returns count [OK]
Hint: LIMIT offset count skips offset, returns count members [OK]
Common Mistakes:
  • Ignoring LIMIT offset and returning from start
  • Confusing score range with member count
  • Including members outside score range
4. You run the command ZRANGEBYSCORE myzset (10 20 but get an error. What is the likely cause?
medium
A. Syntax error due to missing closing parenthesis after 20.
B. Incorrect use of parentheses for exclusive range; missing closing parenthesis.
C. Command is missing the key name.
D. Missing closing parenthesis for exclusive range syntax.

Solution

  1. Step 1: Analyze the command syntax

    The command uses (10 to indicate exclusive minimum score 10, which is correct syntax.

  2. Step 2: Identify the error

    The command ends with 20 without closing parenthesis or space, causing syntax error.

  3. Final Answer:

    Syntax error due to missing closing parenthesis after 20. -> Option A

  4. Quick Check:

    Exclusive range needs parentheses on score values [OK]
Hint: Check parentheses carefully for exclusive score ranges [OK]
Common Mistakes:
  • Forgetting parentheses for exclusive ranges
  • Missing spaces between arguments
  • Using parentheses incorrectly around scores
5. You want to get the top 3 members with scores between 0 and 100 from sorted set myzset, including their scores, but only if their score is greater than 50. Which command achieves this?
hard
A. ZRANGEBYSCORE myzset 50 100 WITHSCORES LIMIT 0 3
B. ZRANGEBYSCORE myzset (50 100 WITHSCORES LIMIT 0 3
C. ZRANGEBYSCORE myzset [50 100 WITHSCORES LIMIT 0 3
D. ZRANGEBYSCORE myzset (50 100 LIMIT 3 WITHSCORES

Solution

  1. Step 1: Define score range with exclusive lower bound

    To get scores greater than 50, use (50 for exclusive minimum.

  2. Step 2: Include scores and limit results

    Use WITHSCORES to include scores and LIMIT 0 3 to get top 3 members.

  3. Step 3: Verify command syntax

    ZRANGEBYSCORE myzset (50 100 WITHSCORES LIMIT 0 3 correctly uses ZRANGEBYSCORE myzset (50 100 WITHSCORES LIMIT 0 3.

  4. Final Answer:

    ZRANGEBYSCORE myzset (50 100 WITHSCORES LIMIT 0 3 -> Option B

  5. Quick Check:

    Exclusive range + WITHSCORES + LIMIT = correct command [OK]
Hint: Use (score for exclusive, WITHSCORES, then LIMIT offset count [OK]
Common Mistakes:
  • Using inclusive 50 instead of exclusive
  • Placing LIMIT before WITHSCORES
  • Using wrong bracket types for exclusive range
Recommended

Ready to prove it?

Pass the test and this pattern moves unknown strong while earning +1 Grip toward your certificate.

10 questions~5 mineasy hard