11_ Redis_ Hyperloglog_ command

Hyperloglog

hyperloglog

What is the cardinality ？

A{ 1,3,5,7,8,7 }
B{ 1,3,5,7,8 }

base （ Elements that don't repeat ） = 5, The error is acceptable !

brief introduction

Redis 2.8.9 The version is updated Hyperloglog data structure !

Redis Hyperloglog The algorithm of cardinality Statistics !

advantage ： The memory occupied is fixed ,2^64 Different elements of Technology , Just waste 12KB Memory ! If you want to compare from the memory point of view Hyperloglog The preferred !

Web page UV( A person visits a website many times , But still count as a person ! ) 0.81% Error rate ! Statistics UV Mission , Negligible !

The traditional way ,set（ Elements are not allowed to repeat ） Save the user's id, And then you can count set The number of elements in as a standard judgment !
This way, if you save a large number of users id, It will be more troublesome ! Our purpose is to count , Instead of saving users id;

127.0.0.1:6379> PFADD mykey a b c d e f g h i j k // Create the first set of elements
(integer) 1
127.0.0.1:6379> PFCOUNT mykey // Statistics mykey The cardinal number of elements
(integer) 11
127.0.0.1:6379> PFADD mykey2 i j k z d e // Create a second set of elements mykey2
(integer) 1
127.0.0.1:6379> PFCOUNT mykey2
(integer) 6
127.0.0.1:6379> PFMERGE mykey3 mykey mykey2 // Combine the two groups mykey mykey2 => mykey3 Combine
OK
127.0.0.1:6379> PFCOUNT mykey3 // Look at the number of unions ！
(integer) 12

If fault tolerance is allowed , Then it must be possible to use Hyperloglog
If fault tolerance is not allowed , Just use set Or your own data type !