Some tests does not work for unprepared statements.
Add `if ActiveRecord::Base.connection.prepared_statements` and fix a
regex for fix tests failure with `prepared_statements: false`.

`initialize_schema_migrations_table` is called in every migrations.

https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/migration.rb#L1080
https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/schema.rb#L51

This means that extra `show variables` is called regardless of the
existence of `schema_migrations` table.

This change is to avoid extra `show variables` if `schema_migrations`
table exists.

Follow up to #22896.

Mysql has a weird bug where it cannot index a string column of utf8mb4 if it is over a certain character limit. To get compatibility with msql we can add a limit to the key column. 191 characters is a very long key, it seems reasonable to limit across all adapters since using a longer key wouldn't be supported in mysql.

Thanks to @kamipo for the original PR and the test refactoring.

Conversation: https://github.com/rails/rails/pull/23009#issuecomment-171416629

Arel handles substitution for bind parameters by now.

Example:

    create_table :posts do |t|
      t.datetime :published_at, default: -> { 'NOW()' }
    end

This is an alternate implementation to #22875, that generalizes a lot of
the logic that type decorators are going to need, in order to have them
work with arrays, ranges, etc. The types have the ability to map over a
value, with the default implementation being to just yield that given
value. Array and Range give more appropriate definitions.

This does not automatically make ranges time zone aware, as they need to
be added to the `time_zone_aware` types config, but we could certainly
make that change if we feel it is appropriate. I do think this would be
a breaking change however, and should at least have a deprecation cycle.

Closes #22875.

/cc @matthewd

uuid-ossp extension is alreadly enabled on test schema.
And `disable_extension!('uuid-ossp', connection)` can be a cause of test failure.
`ActiveRecord::StatementInvalid: PG::UndefinedFunction: ERROR:  function uuid_generate_v1() does not exist`
will happen depending on the execution order.

Avoid instanciate `ActiveRecord::Result` and calling
`ActiveRecord::Result#hash_rows` for the performance.

In MySQL 5.7.5 and up, ONLY_FULL_GROUP_BY affects handling of queries
that use DISTINCT and ORDER BY. It requires the ORDER BY columns in the
select list for distinct queries, and requires that the ORDER BY include
the distinct column.

See https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html

Follow up to #22642.

Since the attributes API is new in Rails 5, we don't actually need to keep
the behavior of `attribute :point`, as it's not a breaking change.

Support passing the schema name as a prefix to table name in
`ConnectionAdapters::SchemaStatements#indexes`. Previously the prefix would
be considered a full part of the index name, and only the schema in the
current search path would be considered.

When this test was run on Windows, the database file would still be in
use, and `File.unlink` would fail. This would cause the temp directory to
be unable to be removed, and error out. By disconnecting the connection
when finished, we can avoid this error.

Fix white-space

Add test case demonstrating flags are received by the adapter

Apart from specific versioning support, our tests should focus on the
behaviour of whatever version they're accompanying, regardless of when
they were written.

Application code should *not* do this.

This removes the following warning which has been out in the case of a PostgreSQL 9.3 below.

```
activerecord/test/cases/adapters/postgresql/geometric_test.rb:265: warning: instance variable @connection not initialized
```

Follow up to #21601.

- key was a poor choice of name. A key implies something that will
  unlock a lock. The concept is actually more like a 'lock identifier'
- mysql documentation calls this a 'lock name'
- postgres documentation calls it a 'lock_id'
- Updated variable names to reflect the preferred terminology for the database in
  question

Reported on #21509, how views is treated by `#tables` are differ
by each adapters. To fix this different behavior, after Rails 5.0
is released, deprecate `#tables`.

And `#table_exists?` would check both tables and views.
To make their behavior consistent with `#tables`, after Rails 5.0
is released, deprecate `#table_exists?`.

This test case was definded by 51de8cee.
`float` and `double` is registered in abstract_mysql_adapter.rb,
we should test not only for mysql adapter, but mysql2 adapter.

- Addresses issue #22092
- Works on Postgres and MySQL
- Uses advisory locks because of two important properties:
  1. The can be obtained outside of the context of a transaction
  2. They are automatically released when the session ends, so if a
  migration process crashed for whatever reason the lock is not left
  open perpetually
- Adds get_advisory_lock and release_advisory_lock methods to database
  adapters
- Attempting to run a migration while another one is in process will
  raise a ConcurrentMigrationError instead of attempting to run in
  parallel with undefined behavior. This could be rescued and
  the migration could exit cleanly instead. Perhaps as a configuration
  option?

Technical Notes
==============

The Migrator uses generate_migrator_advisory_lock_key to build the key
for the lock. In order to be compatible across multiple adapters there
are some constraints on this key.
- Postgres limits us to 64 bit signed integers
- MySQL advisory locks are server-wide so we have to scope to the
  database
- To fulfil these requirements we use a Migrator salt (a randomly
  chosen signed integer with max length of 31 bits) that identifies
  the Rails migration process as the owner of the lock. We multiply
  this salt with a CRC32 unsigned integer hash of the database name to
  get a signed 64 bit integer that can also be converted to a string
  to act as a lock key in MySQL databases.
- It is important for subsequent versions of the Migrator to use the
  same salt, otherwise different versions of the Migrator will not see
  each other's locks.

The first one is quite straightforward. We want to give the proper error
message in the case where a top level constant exists, but we're looking
for a nested one. We just need to port over the change to use
`subclass.name` into these changes.

The second set of failures, which are only present in the mysql adapter
tests, are stranger to me. The failure occurs because we were
previously comparing `subclass.name == self.name` instead of `subclass
== self`. However, I don't think that we need to support creating
anonymous classes which share a table with a class that uses STI,
overrides `name` to return the same name as athe class that we have no
other relationship with, when not assigned to a constant so it could
never be used anyway...

The commits around why that exist give no context, and I think they're
just poorly written tests (WTF does `test_schema` mean anyway, and why
does calling `.first` on some anonymous class test it?). We'll just
disable STI on that class.

This solves the following issue:

```
$ bin/test
Using sqlite3
/Users/senny/Projects/rails/activerecord/test/cases/adapters/mysql2/sp_test.rb:16:in `<class:Mysql2StoredProcedureTest>': undefined method `version' for #<ActiveRecord::ConnectionAdapters::SQLite3Adapter:0x007f8bab4b5b70> (NoMethodError)
	from /Users/senny/Projects/rails/activerecord/test/cases/adapters/mysql2/sp_test.rb:5:in `<top (required)>'
	from /Users/senny/Projects/rails/activesupport/lib/active_support/dependencies.rb:302:in `require'
	from /Users/senny/Projects/rails/activesupport/lib/active_support/dependencies.rb:302:in `block in require'
	from /Users/senny/Projects/rails/activesupport/lib/active_support/dependencies.rb:268:in `load_dependency'
	from /Users/senny/Projects/rails/activesupport/lib/active_support/dependencies.rb:302:in `require'
	from /Users/senny/Projects/rails/railties/lib/rails/test_unit/test_requirer.rb:11:in `block in require_files'
	from /Users/senny/Projects/rails/railties/lib/rails/test_unit/test_requirer.rb:10:in `each'
	from /Users/senny/Projects/rails/railties/lib/rails/test_unit/test_requirer.rb:10:in `require_files'
	from /Users/senny/Projects/rails/railties/lib/rails/test_unit/minitest_plugin.rb:69:in `plugin_rails_init'
	from /Users/senny/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/minitest-5.3.3/lib/minitest.rb:73:in `block in init_plugins'
	from /Users/senny/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/minitest-5.3.3/lib/minitest.rb:71:in `each'
	from /Users/senny/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/minitest-5.3.3/lib/minitest.rb:71:in `init_plugins'
	from /Users/senny/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/minitest-5.3.3/lib/minitest.rb:122:in `run'
	from bin/test:19:in `<main>'
```