The **(11)** does not affect the storage size of the data type, which for an
INT will always be 4 bytes. It affects the **display width**.

http://www.tocker.ca/2015/07/02/proposal-to-deprecate-mysql-integer-display-width-and-zerofill.html

Follow up #21591.

The document of limit option for a text column is incorrect.

MySQL: the limit is byte length, not character length
Pg, Sqlite3: variable unlimited length

autoloading this could possibly cause some weird race condition
when calling an AR::Attribute's singleton method on a threaded server.

Bumps from `5.6` to `5.7`

The last call site of `last_version` was removed with:

838e1832

This method is private API and never used. Let's remove it.

`Schema#migrations_paths` is not supposed to be public API. In fact
it's only used inside `Schema` itself, so let's make it private.

It is always passed in

Usage was removed in 5c449553

This reverts commit d5ba9a42, reversing
changes made to 30c50339.

Reason: This generate the dynalic finders more than one time

[ci skip]

@sgrif can you review when you have time? Thanks!

These 3 methods expect `ConnectionAdapters` to have `tables` method,
so make it clear that `tables` method is interface.

* `ConnectionAdapters::SchemaCache#prepare_tables`
* `db:schema:cache:dump` task
* `SchemaDumper#tables`

Its value never change since associations are defined at class load time
so there is no need to build the hash everytime the method is called.

Before this change:

    Calculating -------------------------------------
             reflections   804.000  i/100ms
    -------------------------------------------------
             reflections      8.213k (±26.2%) i/s -     36.180k

After this change:

    Calculating -------------------------------------
             reflections    24.548k i/100ms
    -------------------------------------------------
             reflections      1.591M (±25.7%) i/s -      7.364M

Benchmark script:

    require 'bundler/setup'

    require 'active_record'
    require 'benchmark/ips'

    ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
    ActiveRecord::Migration.verbose = false

    ActiveRecord::Schema.define do
      100.times do |i|
        create_table "users#{i}", force: true
      end

      create_table :cars, force: true do |t|
        100.times do |i|
          t.references "users#{i}"
        end
      end
    end

    class Car < ActiveRecord::Base
      100.times do |i|
        belongs_to "users#{i}".to_sym
      end
    end

    Benchmark.ips do |x|
      x.report('reflections') { Car.reflections }
    end

Benchmark Script Used:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', path: '~/rails' # master against ref "f1f0a3f8"
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('where with hash single') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with string single') { User.where("users.name = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with hash double') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.", email: "foobar@email.com") }
  x.report('where with string double') { User.where("users.name = ? AND users.email = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.", "foobar@email.com") }
  x.compare!
end
```

Before:
```
Calculating -------------------------------------
where with hash single
                         3.300k i/100ms
where with string single
                         4.965k i/100ms
where with hash double
                         2.594k i/100ms
where with string double
                         4.400k i/100ms
-------------------------------------------------
where with hash single
                         35.161k (± 1.2%) i/s -    178.200k
where with string single
                         53.368k (± 2.9%) i/s -    268.110k
where with hash double
                         27.364k (± 1.1%) i/s -    137.482k
where with string double
                         46.876k (± 2.1%) i/s -    237.600k

Comparison:
where with string single:    53368.1 i/s
where with string double:    46875.5 i/s - 1.14x slower
where with hash single:    35160.8 i/s - 1.52x slower
where with hash double:    27364.0 i/s - 1.95x slower
```

After:
```
Calculating -------------------------------------
where with hash single
                         3.403k i/100ms
where with string single
                         5.167k i/100ms
where with hash double
                         2.659k i/100ms
where with string double
                         4.597k i/100ms
-------------------------------------------------
where with hash single
                         36.410k (± 1.3%) i/s -    183.762k
where with string single
                         55.009k (± 2.6%) i/s -    279.018k
where with hash double
                         27.951k (± 1.4%) i/s -    140.927k
where with string double
                         48.362k (± 2.6%) i/s -    243.641k

Comparison:
where with string single:    55008.6 i/s
where with string double:    48361.5 i/s - 1.14x slower
where with hash single:    36410.1 i/s - 1.51x slower
where with hash double:    27950.9 i/s - 1.97x slower
```

Known failure on Ruby 2.3/trunk: brianmario/mysql2#671

Example:

```ruby
class Person
  include ActiveModel::Validations

  attr_reader :name, :title
  validates_presence_of :name, on: :create
  validates_presence_of :title, on: :update
end

person = Person.new
person.valid?([:create, :update])    # => true
person.errors.messages               # => {:name=>["can't be blank"], :title=>["can't be blank"]}
```

Related with #20418

This reverts commit 51dd2588, reversing
changes made to ecb4e4b2.

This broke Active Record tests

Generic cast-to-text was only added in 8.3.

Benchmark script used:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', path: '~/rails' # master against ref "f1f0a3f8"
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('all') { User.all }
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"

```

Before:
```
Calculating -------------------------------------
                 all    17.569k i/100ms
-------------------------------------------------
                 all    190.854k (± 3.3%) i/s -    966.295k
Total Allocated Object: 85
```

After:
```
Calculating -------------------------------------
                 all    22.237k i/100ms
-------------------------------------------------
                 all    262.715k (± 5.5%) i/s -      1.312M
Total Allocated Object: 80
```

This change allows to instantiate all ActiveRecordError descendant
execption classes without arguments, which might be useful in testing
and is far less surprising than mandatory arguments.

Benchmark Script:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  # gem 'rails', github: 'rails/rails', ref: 'f1f0a3f8'
  gem 'rails', path: '~/rails'
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('where with hash') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with string') { User.where("users.name = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.compare!
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"
```

Stackprof output truncated.
```
TOTAL    (pct)     SAMPLES    (pct)     FRAME
   52  (10.6%)          10   (2.0%)     ActiveRecord::Scoping::Default::ClassMethods#build_default_scope
```

Before:
```
Calculating -------------------------------------
     where with hash     2.789k i/100ms
   where with string     4.407k i/100ms
-------------------------------------------------
     where with hash     29.170k (± 1.9%) i/s -    147.817k
   where with string     46.954k (± 2.7%) i/s -    237.978k

Comparison:
   where with string:    46954.3 i/s
     where with hash:    29169.9 i/s - 1.61x slower

Total Allocated Object: 85
Calculating -------------------------------------
                 all    16.773k i/100ms
-------------------------------------------------
                 all    186.102k (± 3.6%) i/s -    939.288k
```

After:
```
Calculating -------------------------------------
     where with hash     3.014k i/100ms
   where with string     4.623k i/100ms
-------------------------------------------------
     where with hash     31.524k (± 1.3%) i/s -    159.742k
   where with string     49.948k (± 2.3%) i/s -    249.642k

Comparison:
   where with string:    49948.3 i/s
     where with hash:    31524.3 i/s - 1.58x slower

Total Allocated Object: 84
Calculating -------------------------------------
                 all    20.139k i/100ms
-------------------------------------------------
                 all    227.860k (± 2.5%) i/s -      1.148M
```

Stackprof output truncated.
```
TOTAL    (pct)     SAMPLES    (pct)     FRAME
  23   (4.7%)          12   (2.4%)     Hash#transform_keys
  11   (2.2%)          11   (2.2%)     block in Hash#transform_keys
  30   (6.1%)           7   (1.4%)     Hash#stringify_keys
```

Benchmark Script:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', path: '~/rails' # master against ref "f1f0a3f8"
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('where with hash') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with string') { User.where("users.name = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.compare!
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"
```

Before:
```
Calculating -------------------------------------
     where with hash     2.796k i/100ms
   where with string     4.338k i/100ms
-------------------------------------------------
     where with hash     29.177k (± 1.5%) i/s -    148.188k
   where with string     47.419k (± 2.8%) i/s -    238.590k

Comparison:
   where with string:    47419.0 i/s
     where with hash:    29176.6 i/s - 1.63x slower

Total Allocated Object: 85
```

After:
```
Calculating -------------------------------------
     where with hash     2.895k i/100ms
   where with string     4.416k i/100ms
-------------------------------------------------
     where with hash     30.758k (± 2.0%) i/s -    156.330k
   where with string     47.708k (± 2.6%) i/s -    238.464k

Comparison:
   where with string:    47707.9 i/s
     where with hash:    30757.7 i/s - 1.55x slower

Total Allocated Object: 84
```

Many user look `desc` of rake task and they are not familiar with `AR`.
`Active Record` is more familiar word.

Apparently I managed to forget how similar the "tests passing" and
"no status reported" merge indicators look.

Note that the previous `stubs` in test_add_index wasn't working:
the method was still called, and just happened to return false.

See http://www.postgresql.org/docs/9.4/static/sql-dropindex.html
for more details.

Sometimes opts passed in might respond to ==, e.g. `Arel::Nodes::Grouping`. In this case, `opts == :chain` returns `Arel::Nodes::Equality` which causes odd behaviour. Prefer `if :chain == opts` which guarantees that `Symbol#==` would be invoked. Alternatively consider `eql?`.

Current implementation has a lot of utility methods that accept
reflection call a lot of methods on it and exit.

E.g. has_counter_cache?(reflection)

It causes confusion and inability to cache result of the method even
through it always returns the same result for the same reflection
object.
It can be done easier without access to the association context
by moving code into reflection itself.

e.g. reflection.has_counter_cache?

Reflection is less complex object than association so moving code there
automatically makes it simplier to understand.

Since the strings are dynamically computed from a constant, the actual
strings we're creating are a known set. We can compute them ahead of
time, and reduce the number of allocations in that method.

[Rafael Mendonça França + Jean Boussier]