`to_i` was added for SQLite3 adapter which did not handle number
but sqlite3 gem already supports it then `to_i` is unnecessary.

else condition is kept for adapters which return string,
i.e. mysql(not mysql2) and sqlserver.

Renamed `test_cache_does_not_wrap_string_results_in_arrays`
to `test_cache_does_not_wrap_results_in_arrays` to explain the
current behavior. most of adapters return integer, not only string.

* Refer these commits:

"future proofing the sqlite3 adapter code"
https://github.com/rails/rails/commit/beda2d43d6ac5c3435fc2fba0cbd108c20fe1c67

"Refactor calculation test to remove unneeded SQLite special case."
https://github.com/rails/rails/commit/47d568ed3fc701934ebe80b276f3d8bf6951c93f

"no need to to_i, sqlite does that for us"
https://github.com/rails/rails/commit/6cf44a1bd64ba10497742d70ad78fe68faa16e99

Fixes #30496.

when tested with MariaDB 10.2.8 or higher

Refer #30485

https://mariadb.com/kb/en/the-mariadb-library/alter-table/#drop-column-if-exists-col_name-cascaderestrict
> MariaDB starting with 10.2.8
> Dropping a column that is part of a multi-column UNIQUE constraint is not permitted.

Don't use remove_method or remove_possible_method just before a new
definition: at best the purpose is unclear, and at worst it creates a
race condition.

Instead, prefer redefine_method when practical, and
silence_redefinition_of_method otherwise.

Without this, `db:structure:dump` task raises an error as follwing:

```
can't modify frozen String
activerecord/lib/active_record/tasks/sqlite_database_tasks.rb:77:in `run_cmd_error'
activerecord/lib/active_record/tasks/sqlite_database_tasks.rb:72:in `run_cmd'
activerecord/lib/active_record/tasks/sqlite_database_tasks.rb:52:in `structure_dump'
activerecord/lib/active_record/tasks/database_tasks.rb:219:in `structure_dump'
activerecord/lib/active_record/railties/databases.rake:279:in `block (3 levels) in <main>'
railties/lib/rails/commands/rake/rake_command.rb:23:in `block in perform'
railties/lib/rails/commands/rake/rake_command.rb:20:in `perform'
railties/lib/rails/command.rb:48:in `invoke'
railties/lib/rails/commands.rb:18:in `<main>'
```

- Closes #30441

If `:readers` fixture is loaded before the test, it will be failed.
Use `firm.developer_ids` instead because we don't have `:contracts`
fixture for now.

https://travis-ci.org/rails/rails/jobs/268976230#L729

Because `add_as_polymorphic_through` is only called when
`options[:source_type]` is true.

`RuntimeReflection#alias_candidate` is no longer used since 0408e212.

Context: https://github.com/rails/rails/pull/29619#discussion_r125158589

I was added a table options after `force: :cascade` in #17569 for not
touching existing tests (reducing diff). But `force: :cascade` is not an
important information. So I prefer to place a table options before
`force: :cascade`.

https://travis-ci.org/rails/rails/jobs/268599781#L784

This regression was caused by caa178c1. The block for
`set_inverse_instance` should also be passed to join dependency.

Fixes #30402.

when other `AutomaticInverseFindingTests` load `:comments` fixture
but does not load `:posts`.

Refer #30385 for similar issue

There were similar pull requests #26370 #27575 fixed by different way
by moving `require "models/post"` before `require "models/comment"`

`:authors` has a foreign key to `:author_addresses`.
If only `:authors` fixture loaded into the database which supports
foreign key and checks the existing data when enabling foreien keys
like Oracle, it raises the following error

`ORA-02298: cannot validate (ARUNIT.FK_RAILS_94423A17A3) - parent keys not found`

It is because there is no parent data exists in `author_addresses` table.

Here are how other database with foreign key support works:

- MySQL does not check the existing data when enabling foreign key by `foreign_key_checks=1`
https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_foreign_key_checks
> Setting foreign_key_checks to 1 does not trigger a scan of the existing table data. Therefore, rows added to the table while foreign_key_checks=0 will not be verified for consistency.

- PostgreSQL database itself has a feature to check existing data when
enabling foreign key and discussed at #27636, which is reverted.

Currently `SchemaDumper` is only customizable for column options. But
3rd party connection adapters (oracle-enhanced etc) need to customizable
for table or index dumping also. To make it possible, I introduced
adapter specific `SchemaDumper` classes for that.

ActiveRecord associations automatically guess the inverse associations.
But this feature does not work correctly on assoctions for STI.
For example, before this commit

```
class Post < ActiveRecord::Base
  belongs_to :author
end

class SpecialPost < Post; end

class Author < ActiveRecord::Base
  has_many :posts
  has_many :special_posts
end
```

`author.posts.first.author` works correctly, but
`author.special_posts.first.author` does not work correctly.

`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]]
```