Hi, fellows!
Sometimes we need to get a random record from some table. The common way to
get it is to use Array#sample method on a relation. Or, if your table is
quiet large, you can use one of the ways your DB provides to do such thing.
It could be
MyModel.order('RAND()').limit(1)
if you use MySQL, or
MyModel.order('RANDOM()').limit(1)
if you use Postgresql.
This way is much more fast than using #sample method, even if your table is
less then 1000 rows. But it has two downsides.
First, this code is now database-dependent. So if your code will some day
migrate on an another DB, you will have to change it.
Second, it is not obvious for some rails novices, that doing
`MyModel.all.sample` you actually use Array#sample method, that loads all
relation collection in memory. If rails tend to be easy-to-start and clear
for developer, it should not act this way.
So, my suggestion is to implement #sample method somewhere in ActiveRecord
so that it will generate database query to get random row. There are
functions to do it in all common DB distributions (MySQL, Postgresql, MS
SQL, IBM DB2, Oracle). The usage could look like this:
MyModel.sample # returns one random record
MyModel.sample(10) # returns 10 random records
MyModel.all.sample # works identical to the first two examples; and this is
not the Array#sample method.
MyModel.some_scope.sample # returns one random record from a scope
and so on.
I think the performance benefit from such enhancement is quite obvious, but
anyway, here is a benchmark for a Postgres table with 180 000 rows.
2.1.5 (main):0 > Benchmark.bm do |x|
2.1.5 (main):0 * x.report('sample') { LessonVersion.all.sample }
2.1.5 (main):0 * x.report('RANDOM()') {
LessonVersion.order('RANDOM()').limit(1) }
2.1.5 (main):0 * end
user system total real
sample 9.810000 8.690000 18.500000 ( 29.282437)
RANDOM() 0.000000 0.000000 0.000000 (0.000187)
=> [
[0] #<Benchmark::Tms:0x007fe7c699c568 @label="sample", @real=29.282437,
@cstime=0.0, @cutime=0.0, @stime=8.69, @utime=9.81, @total=18.5>,
[1] #<Benchmark::Tms:0x007fe7c69a4420 @label="RANDOM()", @real=0.000187,
@cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.0, @total=0.0>
]
I've already set up rails-dev-box on my laptop and cloned rails repository,
but there is a CONTRIBUTING.md file, that recommends to get positive
feedback before writing code. So, here is my proposal. I think I can write
it myself, but ofcourse implementation suggestions are very welcome.
Especially since I'm not very comfortible with ActiveRecord sources yet.
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