`select_values` is a local variable defined at previous line.
`select_values += select_values` is totally useless.

`valid_type?` should return true if a type exists in
`native_database_types` at least.

https://github.com/rails/rails/blob/v5.1.0.beta1/activerecord/lib/active_record/schema_dumper.rb#L136

Currently `change_column` cannot drop default if `null: false` is
specified at the same time. This change fixes the issue.

```ruby
  # cannot drop default
  change_column "tests", "contributor", :boolean, default: nil, null: false

  # we need the following workaround currently
  change_column "tests", "contributor", :boolean, null: false
  change_column "tests", "contributor", :boolean, default: nil
```

Closes #26582

`DISTINCT` clause is applied inside aggregate function by
`operation_over_aggregate_column` if needed. Unneeded outside aggregate
function.

```ruby
  # Before
  author.unique_categorized_posts.count
  # => SELECT DISTINCT COUNT(DISTINCT "posts"."id") FROM "posts" INNER JOIN "categorizations" ON "posts"."id" = "categorizations"."post_id" WHERE "categorizations"."author_id" = ?  [["author_id", 2]]

  # After
  author.unique_categorized_posts.count
  # => SELECT COUNT(DISTINCT "posts"."id") FROM "posts" INNER JOIN "categorizations" ON "posts"."id" = "categorizations"."post_id" WHERE "categorizations"."author_id" = ?  [["author_id", 2]]
```

Closes #27615

Since using a `ActiveSupport::Deprecation::DeprecatedConstantProxy`
would prevent people from inheriting this class and extending it
from the `ActiveSupport::HashWithIndifferentAccess` one would break
the ancestors chain, that's the best option we have here.

Remove styling from `true` and `false`.

Originally `quoted_id` was used in legacy quoting mechanism. Now we use
type casting mechanism for that. Let's deprecate `quoted_id`.

Fixes #26556.

The native timestamp type in MySQL is different from datetime type.
Internal representation of the timestamp type is UNIX time, This means
that timestamp columns are affected by time zone.

```
> SET time_zone = '+00:00';
Query OK, 0 rows affected (0.00 sec)

> INSERT INTO time_with_zone(ts,dt) VALUES (NOW(),NOW());
Query OK, 1 row affected (0.02 sec)

> SELECT * FROM time_with_zone;
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2016-02-07 22:11:44 | 2016-02-07 22:11:44 |
+---------------------+---------------------+
1 row in set (0.00 sec)

> SET time_zone = '-08:00';
Query OK, 0 rows affected (0.00 sec)

> SELECT * FROM time_with_zone;
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2016-02-07 14:11:44 | 2016-02-07 22:11:44 |
+---------------------+---------------------+
1 row in set (0.00 sec)
```

[Arthur Nogueira Neves & Matthew Draper]

This ensures multiple threads inside a transactional test to see consistent
database state.

When a system test starts Puma spins up one thread and Capybara spins up
another thread. Because of this when tests are run the database cannot
see what was inserted into the database on teardown. This is because
there are two threads using two different connections.

This change uses the statement cache to lock the threads to using a
single connection ID instead of each not being able to see each other.
This code only runs in the fixture setup and teardown so it does not
affect real production databases.

When a transaction is opened we set `lock_thread` to `Thread.current` so
we can keep track of which connection the thread is using. When we
rollback the transaction we unlock the thread and then there will be no
left-over data in the database because the transaction will roll back
the correct connections.

[ Eileen M. Uchitelle, Matthew Draper ]

Pointed out by @matthewd that the HWIA subclass changes the
AS scoped class and top-level HWIA hierarchies out from under
existing classes.

This reverts commit 71da3909, reversing
changes made to 41c33bd4.

This constant was kept for the sake of backward compatibility; it
is still available under `ActiveSupport::HashWithIndifferentAccess`.

Furthermore, since Ruby 2.5 (https://bugs.ruby-lang.org/issues/11547)
won't support top level constant lookup, people would have to update
their code anyway.

Extra `data_source_exists?(table_name)` is unneeded if
`drop_table(table_name, if_exists: true)` directly.

Fixes CI failure caused by #27961.

https://travis-ci.org/rails/rails/jobs/201472146#L2390-L2409

primary_keys(table) needs to query various metadata tables in Postgres
to determine the primary key for the table. Previously, it did so using
a complex common table expression against pg_constraint and
pg_attribute.

This patch simplifies the query by using information_schema tables.
This simplifies the logic, making the query far easier to understand,
and additionally avoids an expensive unnest, window function query, and
common table expression.

Add `default_index_type?` to the generic schema dumper doesn't have the knowledge about an index type

The `default` arg of `index_name_exists?` is only used the adapter does
not implemented `indexes`. But currently all adapters implemented
`indexes` (See #26688). Therefore the `default` arg is never used.

Closes #27980

Closes #27979

`supports_primary_key?` was added to determine if `primary_key` is
implemented in the adapter in f0602214. But we already use `primary_key`
without `supports_primary_key?` (207f266c, 5f3cf424) and using
`supports_primary_key?` has been removed in #1318. This means that
`supports_primary_key?` is no longer used in the internal and Active
Record doesn't work without `primary_key` is implemented (all adapters
must implement `primary_key`).

Closes #27977

These files are not using `strip_heredoc`.

Closes #27976

So queries are not run against the previous table name.

Closes #27953

This was never really intended to work (at least not without calling
`define_attribute_methods`, which is less common with Active Record). As
we move forward the intention is to require the use of `attribute` over
`attr_accessor` for more complex model behavior both on Active Record
and Active Model, so this behavior is deprecated.

Fixes #27956.
Close #27963.

[Alex Serban & Sean Griffin]

Currently schema dumper lost the unsigned option when primary key is
defined as bigint with unsigned. This commit fixes the issue.

Closes #27960

This reverts commit d6529af2.

primary_keys(table) needs to query various metadata tables in Postgres to
determine the primary key for the table. Previously, it did so using a
complex common table expression against pg_constraint and pg_attribute.

This patch simplifies the query by joining pg_index against pg_attribute
instead of going through pg_constraint. This avoids an expensive unnest,
window function query, and common table expression.

EXPLAINing these queries in Postgres against a database with a single
table with a composite primary key shows a 66% reduction in the plan and
execute latencies. This is significant during application startup time,
especially against very large schemas, where these queries would be even
slower and more numerous.

Closes #27949

Column options are passed as an hash args then used as `options` hash in
`add_column_options!`. Converting args to attributes is inconvinient for
using options as an hash.