This commit speeds up allocating homogeneous lists of AR objects. We
can know if the result set contains an STI column before initializing
every AR object, so this change pulls the "does this result set contain
an STI column?" test up, then uses a specialized instantiation function.
This way we only have to check for an STI column once rather than N
times.
This change also introduces a new initialization function that is meant
for use when allocating AR objects that come from the database. Doing
this allows us to eliminate one hash allocation per AR instance.
Here is a benchmark:
```ruby
require 'active_record'
require 'benchmark/ips'
ActiveRecord::Base.establish_connection adapter: "sqlite3", database: ":memory:"
ActiveRecord::Migration.verbose = false
ActiveRecord::Schema.define do
create_table :users, force: true do |t|
t.string :name
t.timestamps null: false
end
end
class User < ActiveRecord::Base; end
2000.times do
User.create!(name: "Gorby")
end
Benchmark.ips do |x|
x.report("find") do
User.limit(2000).to_a
end
end
```
Results:
Before:
```
[aaron@TC activerecord (master)]$ be ruby -I lib:~/git/allocation_tracer/lib speed.rb
Warming up --------------------------------------
find 5.000 i/100ms
Calculating -------------------------------------
find 56.192 (± 3.6%) i/s - 285.000 in 5.080940s
```
After:
```
[aaron@TC activerecord (homogeneous-allocation)]$ be ruby -I lib:~/git/allocation_tracer/lib speed.rb
Warming up --------------------------------------
find 7.000 i/100ms
Calculating -------------------------------------
find 72.204 (± 2.8%) i/s - 364.000 in 5.044592s
```
The Enumerator#size method was introduced in Ruby 2.0.
These tests were added when Rails 4.1 was current, and Ruby 1.9.3 was
still supported. Since Rails 5 only Ruby >= 2.2.2 is supported, so the
checks are no longer necessary.
Attempting to reduce the number of places that care about the details of
how type casting occurs. We remove the type casting of the primary key
in `JoinDependecy`, rather than encapsulating it. It was originally
added for consistency with
40898c8c19 (diff-06059df8d3dee3101718fb2c01151ad0R211),
but that conditional was later removed in
d7ddaa530f.
What is important is that the same row twice will have the same value
for the primary key, which it will.
