CODEWITHSUNDEEP
redis
tabreaz
tabreaz

Reputation: 659

Redis Sorted Set ZSCAN order of items

I have a sorted set in Redis with timestamp and different type of relationships to the set is stored together.

Example dataset is explained below:

zadd s1 10 rel1:val1  
zadd s1 15 rel1:val2
zadd s1 12 rel1:val3  

zadd s1 10 rel2:v1  
zadd s1 12 rel2:v2
zadd s1 5 rel1:v3

What I'm looking for top element of rel1 or rel2 from the set, if I tried to store them separately I will end with many keys, which I want to avoid.

I tried ZSCAN and the results are in asc order by score like below

localhost:6379> zscan s1 0 match "rel1*" count 10
1) "0"
2) 1) "rel1:v3"
   2) "5"
   3) "rel1:val1"
   4) "10"
   5) "rel1:val3"
   6) "12"
   7) "rel1:val2"
   8) "15"

And if I score the timestamp in -ive I get the results as expected top element

localhost:6379> zadd s1 -10 rel1:val1
(integer) 0
localhost:6379> zadd s1 -20 rel1:val2
(integer) 0
localhost:6379> zadd s1 -30 rel1:val3
(integer) 0
localhost:6379> zscan s1 0 match "rel1*" count 10
1) "0"
2) 1) "rel1:val3"
   2) "-30"
   3) "rel1:val2"
   4) "-20"
   5) "rel1:val1"
   6) "-10"
   7) "rel1:v3"
   8) "5"

My question can I relay on this results and believe the results are always in asc order with ZSCAN.

I can't use ZREVRANGE or ZRANGE commands as I have to get top n items with MATCHING the members.

Upvotes: 1

Views: 3573

Answers (2)

Tao Chen
Tao Chen

Reputation: 1

If you use small sample data like the given one, you might probably get the correctly ordered results. But don't trust it.

Just as @for_stack said, ZSCAN doesn't keep the order of results. You could generate a large number of samples to test it, then you will find. You have to switch to ZRANGE, ZREVRANGE or other ways.

Upvotes: 0

for_stack
for_stack

Reputation: 22906

NO. You cannot rely on it. The order is undefined.

As we known, ZSET is implemented with a dict/hash, and a skiplist. ZSCAN command scans the dict, not the skiplist. Since dict is unordered, the order of scan result is undefined.

Upvotes: 2

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