Expert · Module 27 · Lesson 2 of 5
Redis sorted sets and skip lists
Inside Redis sorted sets: why ZSCORE is O(1) while ZRANK is O(log n), how the listpack-to-skip-list conversion works, and how randomly rolled levels make ranks cheap without any rebalancing.
By the end you can
- State why a sorted set keeps both a member dict and a skip list, and name the operations each one serves
- Trace a ZRANK walk through the levels and compute the rank it returns
- Relate the level-promotion probability to the O(log n) expected cost of rank and range queries
Builds on Red-black trees, Choosing an ordered structure.
01 · A question first
ZADD one member and Redis updates two structures at once. ZSCORE on a set with ten million members returns instantly, while ZRANK walks a chain of pointers whose levels were rolled with a die at insert time. Nothing in the structure is ever rotated or rebalanced — so what keeps the walk short?
Make a guess before you read on.
02 · The idea
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What's inside
- One command, two structures
- The bottom level and the levels above it
- The trace above: four hops to a rank
- Why random levels beat rotations
- What each command costs
- Pseudocode & Multi-Language Implementations (C++, Java, Python)
- Complexity Derivation, Edge Cases & Common Mistakes
03 · Practice
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