`SET time zone 'value'` is an alias for `SET timezone TO 'value'`.

https://www.postgresql.org/docs/current/static/sql-set.html

So if `variables["timezone"]` is specified, it is enough to
`SET timezone` once.

This is only used for the internal `column_spec` and
`column_spec_for_primary_key`.

currently integer types extracts the `limit` from `sql_type`. But the
lookup key of type map is the `oid` in postgresql adapter. So in most
case `sql_type` is passed to `extract_limit` as `""` and `limit` is
extracted as `nil`.

https://github.com/rails/rails/blob/v5.1.0.beta1/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L445

In mysql2 adapter, `limit` is registered correctly without extracting
from `sql_type`.

https://github.com/rails/rails/blob/v5.1.0.beta1/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb#L678-L682

Postgresql adapter should also be registered correctly.

``` ruby
  conn = ActiveRecord::Base.connection
  conn.select_all("SELECT 1::smallint, 2::integer, 3::bigint").column_types.map do |name, type|
    [name, type.limit]
  end
```

Before:

``` ruby
  # => [["int2", nil], ["int4", nil], ["int8", nil]]
```

After:

``` ruby
  # => [["int2", 2], ["int4", 4], ["int8", 8]]
```

* Test `test_unabstracted_database_dependent_types` for `PostgreSQLAdapter`
* Add `test_change_column_with_new_precision_and_scale` for `SQLite3Adapter`
 * This test case and comment was lost at 28bb02a7

Otherwise random CI failure will be caused.

https://travis-ci.org/rails/rails/jobs/265848046#L777

Since 213796fb, it was lost the ability that SQL with binds for `insert`,
`update`, and `delete` (like `select_all`). This restores the ability
because `insert`, `update`, and `delete` are public API, so it should
not be removed without deprecation.

I tried to change the expectation in #29976, but it is expected behavior
at least for now. So I added the test cases to prevent anyone change the
expectation.

Because `to_sql` is public API. I introduced `to_sql_and_binds` internal
API to return SQL and binds.

`where.not` with multiple conditions is still unexpected behavior. But
`where.not` with only polymorphic association has already been fixed in
213796fb.

Closes #14161.
Closes #16983.
Closes #17010.
Closes #26207.

This commit addresses these failures when the backend RDBMS executes implicit commit for DDL like MySQL and Oracle.

```ruby
$ ARCONN=mysql2 bin/test test/cases/integration_test.rb test/cases/timestamp_test.rb --seed 58225 -n '/^(?:TimestampTest#(?:test_index_is_created_for_both_timestamps)|IntegrationTest#(?:test_cache_key_for_newer_updated_at|test_cache_key_format_for_existing_record_with_updated_at|test_cache_key_format_for_existing_record_with_updated_at_and_custom_cache_timestamp_format))$/' -v
Using mysql2
Run options: --seed 58225 -n "/^(?:TimestampTest#(?:test_index_is_created_for_both_timestamps)|IntegrationTest#(?:test_cache_key_for_newer_updated_at|test_cache_key_format_for_existing_record_with_updated_at|test_cache_key_format_for_existing_record_with_updated_at_and_custom_cache_timestamp_format))$/" -v

TimestampTest#test_index_is_created_for_both_timestamps = 0.19 s = .
IntegrationTest#test_cache_key_format_for_existing_record_with_updated_at = 0.05 s = F
IntegrationTest#test_cache_key_format_for_existing_record_with_updated_at_and_custom_cache_timestamp_format = 0.02 s = F
IntegrationTest#test_cache_key_for_newer_updated_at = 0.01 s = F

Finished in 0.272880s, 14.6585 runs/s, 14.6585 assertions/s.

  1) Failure:
IntegrationTest#test_cache_key_format_for_existing_record_with_updated_at [/home/yahonda/git/rails/activerecord/test/cases/integration_test.rb:111]:
--- expected
+++ actual
@@ -1 +1 @@
-"developers/1-20170717135609430848"
+"developers/1-20170817135609283207"

  2) Failure:
IntegrationTest#test_cache_key_format_for_existing_record_with_updated_at_and_custom_cache_timestamp_format [/home/yahonda/git/rails/activerecord/test/cases/integration_test.rb:116]:
--- expected
+++ actual
@@ -1 +1 @@
-"cached_developers/1-20170717135609"
+"cached_developers/1-20170817135609"

  3) Failure:
IntegrationTest#test_cache_key_for_newer_updated_at [/home/yahonda/git/rails/activerecord/test/cases/integration_test.rb:147]:
--- expected
+++ actual
@@ -1 +1 @@
-"developers/1-20170717145609430848"
+"developers/1-20170817135609283207"

4 runs, 4 assertions, 3 failures, 0 errors, 0 skips
$
```

`ActiveRecord::TestCase::TimestampTest#test_index_is_created_for_both_timestamps`
calls`ActiveRecord::TestCase::TimestampTest#setup` which updates `updated_at` column value.

```ruby
    @developer = Developer.first
    ... snip ...
    @developer.update_columns(updated_at: Time.now.prev_month)
```

This transaction expected to be rolled back, but if the backend RDBMS like MySQL perfomes
implicit commit when DDL executed, this transacion is committed by `create table` statement
inside `ActiveRecord::TestCase::TimestampTest#test_index_is_created_for_both_timestamps`.

It causes these failures above which expect `update_at` column value are not changed(rollbacked).

[ci skip]

And enable `context_dependent` of Style/BracesAroundHashParameters cop.

Since c51f9b61 changed the "_before_type_cast" expectation for enum.

`write_attribute_without_type_cast` is defined as a private method in
`AttributeMethods::Write`, but `AttributeMethods::Dirty` overrode it as
a public method. It should be kept the original visibility.

Follow up of #29834.

Fixes #30266.

Add missing backticks
Add missing *
Add missing .

When using a has_many through relation and then summing an attribute
the distinct was not being used. This will ensure that when summing
an attribute, the number is only used once when distinct has been used.

`Firm.id` is a bigint if mysql2 adapter is used, but `firm_id` is an
integer. It will cause an out of range error.

https://travis-ci.org/rails/rails/jobs/264112814#L776
https://travis-ci.org/rails/rails/jobs/264112835#L919

The primary key on the owner record is abstracted as `join_foreign_key`.

`source_type_info` is only used for `constraints` in
`PolymorphicReflection`.

We already have a test case for `serialize` with a custom coder in
`PostgresqlHstoreTest`.

https://github.com/rails/rails/blob/v5.1.3/activerecord/test/cases/adapters/postgresql/hstore_test.rb#L316-L335

https://travis-ci.org/rails/rails/jobs/263617099#L769-L775

Since `Relation` includes `Enumerable`, it is enough to use `super`
simply.

It is incorrect to treat `options[:primary_key]` as
`association_primary_key` if `has_many` associations because the
`:primary_key` means the column on the owner record, not on the
association record. It will break `ids_reader` and `ids_writer`.

```ruby
  people(:david).essay_ids
  # => ActiveRecord::StatementInvalid: Mysql2::Error: Unknown column 'essays.first_name' in 'field list': SELECT `essays`.first_name FROM `essays` WHERE `essays`.`writer_id` = 'David'
```

Fixes #14439.

The previous paragraph mentions that you can hand off the same processing
queue to multiple workers. This completes the following example below it.

It can use `AbstractReflection#table_name` simply.

This comment was added at 97849deb, but `AssociationProxy` and
`test_triple_equality` was removed at 1644663b. Currently the `===` is
used for `test_decorated_polymorphic_where` that added at #11945.
So I updated "association proxies" to "decorated models".

And also, currently `Core::ClassMethods` appears in the doc.

http://api.rubyonrails.org/classes/ActiveRecord/Core/ClassMethods.html

But it looks like that the methods in the module is not public API.
So I also added `# :nodoc:` to the module.

Fixes #29045.

refs: https://github.com/rails/rails/pull/30161

```
$ echo "+@size+" | rdoc --pipe
<p>+@size+</p>

$ echo "<tt>@size</tt>" | rdoc --pipe
<p><code>@size</code></p>
```

[ci skip]

Because `JSONSharedTestCases` is also used for `Mysql2JSONTest`.