Top-N queries in Redis - Step-by-Step Execution

Concept Flow - Top-N queries

Start with dataset

↓

Sort data by score or value

↓

Select top N items

↓

Return top N results

↓

End

Top-N queries sort data by a score or value and then pick the top N items to return.

Execution Sample

Redis

ZADD leaderboard 100 "Alice" 200 "Bob" 150 "Carol"
ZREVRANGE leaderboard 0 2 WITHSCORES

Add scores for players and get top 3 players with their scores.

Execution Table

Step	Command	Action	Data State	Output
1	ZADD leaderboard 100 "Alice"	Add Alice with score 100	{"leaderboard": {"Alice": 100}}	OK
2	ZADD leaderboard 200 "Bob"	Add Bob with score 200	{"leaderboard": {"Alice": 100, "Bob": 200}}	OK
3	ZADD leaderboard 150 "Carol"	Add Carol with score 150	{"leaderboard": {"Alice": 100, "Bob": 200, "Carol": 150}}	OK
4	ZRANGE leaderboard -3 -1 WITHSCORES	Get all 3 members sorted by score ascending	{"leaderboard": {"Alice": 100, "Bob": 200, "Carol": 150}}	["Alice", "100", "Carol", "150", "Bob", "200"]
5	ZREVRANGE leaderboard 0 2 WITHSCORES	Get top 3 members sorted by score descending	{"leaderboard": {"Alice": 100, "Bob": 200, "Carol": 150}}	["Bob", "200", "Carol", "150", "Alice", "100"]
6	End	Top-N query complete	{"leaderboard": {"Alice": 100, "Bob": 200, "Carol": 150}}	Top 3 players with scores returned

💡 All top 3 players retrieved; query ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
leaderboard	{}	{"Alice": 100}	{"Alice": 100, "Bob": 200}	{"Alice": 100, "Bob": 200, "Carol": 150}	{"Alice": 100, "Bob": 200, "Carol": 150}

Key Moments - 2 Insights

Why does ZRANGE with -3 -1 return all players sorted ascending, not top scores?

ZRANGE returns elements in ascending order by score. To get top scores descending, use ZREVRANGE as shown in step 5.

How does Redis store scores and members in sorted sets?

Redis stores members with their scores in a sorted set, allowing fast retrieval by score order, as seen in the leaderboard variable changes.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what order are the players returned?

A Descending by score: Bob, Carol, Alice

B Random order

C Ascending by score: Alice, Carol, Bob

D Alphabetical order

💡 Hint

Check the output column at step 4 showing scores from 100 to 200.

At which step does the leaderboard contain all three players?

A After Step 1

B After Step 3

C After Step 2

D After Step 4

💡 Hint

Look at variable_tracker for leaderboard state after each step.

If you want the top 2 players by highest score, which command would you use?

A ZREVRANGE leaderboard 0 1

B ZRANGE leaderboard 0 1

C ZRANGE leaderboard -2 -1

D ZREVRANGE leaderboard -2 -1

💡 Hint

Refer to step 5 where ZREVRANGE returns highest scores first.

Concept Snapshot

Top-N queries in Redis use sorted sets.
Add members with scores using ZADD.
Use ZREVRANGE to get top N by highest scores.
ZRANGE returns sorted ascending by score.
Top-N queries help find best scoring items quickly.

Full Transcript

This visual execution shows how Redis handles Top-N queries using sorted sets. We add players with scores using ZADD commands. The leaderboard variable tracks the stored data. Using ZRANGE returns members sorted by score ascending, while ZREVRANGE returns them descending, which is useful for Top-N queries. The execution table traces each command, data state, and output. Key moments clarify common confusions about sorting order and data storage. The quiz tests understanding of order, data state, and correct commands for Top-N retrieval.