Database (e.g. MySql) :order=>'rand()' is slow.
use http://jan.kneschke.de/projects/mysql/order-by-rand/ to replace ORDER BY RAND()
Now let's see what happends to our performance. We have 3 different queries for solving our problems.
- Q1. ORDER BY RAND()
- Q2. RAND() * MAX(ID)
- Q3. RAND() * MAX(ID) + ORDER BY ID
Q1 is expected to cost N * log2(N), Q2 and Q3 are nearly constant.
The get real values we filled the table with N rows ( one thousand to one million) and executed each query 1000 times.
100 1.000 10.000 100.000 1.000.000 Q1 0:00.718s 0:02.092s 0:18.684s 2:59.081s 58:20.000s Q2 0:00.519s 0:00.607s 0:00.614s 0:00.628s 0:00.637s Q3 0:00.570s 0:00.607s 0:00.614s 0:00.628s 0:00.637s As you can see the plain ORDER BY RAND() is already behind the optimized query at only 100 rows in the table.
include FastRandom in model
class Post < ActiveRecord::Base include FastRandom end
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