`column_types` is empty except PostgreSQL adapter, and
`attribute_types.each_key { |k| column_types.delete k }` is also empty
even if PostgreSQL adapter almost all case, so that code is quite
useless. This improves performance for `find_by_sql` to avoid that
useless loop as much as possible.

```ruby
ActiveRecord::Schema.define do
  create_table :active_storage_blobs do |t|
    t.string   :key,          null: false
    t.string   :filename,     null: false
    t.string   :content_type
    t.text     :metadata
    t.string   :service_name, null: false
    t.bigint   :byte_size,    null: false
    t.string   :checksum,     null: false
    t.datetime :created_at,   null: false

    t.index [ :key ], unique: true
  end
end

class ActiveStorageBlob < ActiveRecord::Base
end

Benchmark.ips do |x|
  x.report("find_by") { ActiveStorageBlob.find_by(id: 1) }
end
```

Before:

```
Warming up --------------------------------------
             find_by     1.256k i/100ms
Calculating -------------------------------------
             find_by     12.595k (± 3.4%) i/s -     64.056k in   5.091599s
```

After:

```
Warming up --------------------------------------
             find_by     1.341k i/100ms
Calculating -------------------------------------
             find_by     13.170k (± 3.5%) i/s -     67.050k in   5.097439s
```

To avoid column types loop for PostgreSQL adapter, this skips returning
additional column types if a column has already been type casted by pg
decoders. Fortunately this fixes #36186 partly for common types.

https://buildkite.com/rails/rails/builds/68559#2decc846-c6b5-46f7-a805-426a8063d36c/1016-1027

Relying on the `Arel::Table.engine` is convenient if an app have only a
single kind of database, but if not so, the global state is not always
the same with the current connection.

`allowed_index_name_length` was used for internal temporary operations
in SQLite3, since index name in SQLite3 must be globally unique and
SQLite3 doesn't have ALTER TABLE feature (so it is emulated by creating
temporary table with prefix).

`allowed_index_name_length` was to reserve the margin for the prefix,
but actually SQLite3 doesn't have a limitation for identifier name
length, so the margin has removed at 36901e6e.

Now `allowed_index_name_length` is no longer relied on by any adapter,
so I'd like to remove the internal specific method which is no longer
used.

Define `value_for_database` and `value_before_type_cast` methods, and
use those.

`in_clause_length` was added at c5a284f8 to address to Oracle IN clause
length limitation.

Now `in_clause_length` is entirely integrated in Arel visitor since
#35838 and #36074.

Since Oracle visitors are the only code that rely on `in_clause_length`.
so I'd like to remove that from Rails code base, like has removed Oracle
visitors (#38946).

Skip test cases for upsert_all on relation if database adapter doesn't support update on duplicate records

If only one Arel node exist, wrapping a node by `And` node is obviously
redundant, make concise Arel ast will improve performance for visiting
the ast (about 20% faster for complex ast case).

```ruby
class Post < ActiveRecord::Base
end

posts = (0..500).map { |i| Post.where(id: i) }

Benchmark.ips do |x|
  x.report("inject scopes") { posts.inject { |res, scope| res.or(scope) }.to_sql }
end
```

Before:

```
Warming up --------------------------------------
      where with ids     8.000  i/100ms
Calculating -------------------------------------
      where with ids     80.416  (± 2.5%) i/s -    408.000  in   5.078118s
```

After:

```
Warming up --------------------------------------
      where with ids     9.000  i/100ms
Calculating -------------------------------------
      where with ids     96.126  (± 2.1%) i/s -    486.000  in   5.058960s
```

Currently, `count` and `average` always returns numeric value, but
`sum`, `maximum`, and `minimum` not always return numeric value if
aggregated on custom attribute type.

I think that inconsistent behavior is surprising:

```ruby
# All adapters except postgresql adapter are affected
# by custom type casting.

Book.group(:status).sum(:status)
# => { "proposed" => "proposed", "published" => nil }
```

That is caused by fallback looking up cast type to `type_for(column)`.
Now all supported adapters can return numeric value without that
fallback, so I think we can remove that, it will also fix aggregate
functions to return numeric value consistently.

This allows to work `author.books.insert_all` as expected.
Co-authored-by: NJosef Šimánek <josef.simanek@gmail.com>

mysql2 gem have type casting mechanism from raw payload to Ruby
primitive object.

There is no worth to keep those unused aliases and tests which are
private API.

reload column_names after ignored_columns assignment

code review: remove unnecessary setup and move up reload_schema_from_cache

Co-authored-by: NRyuta Kamizono <kamipo@gmail.com>

Update activerecord changelog

Specify DB name in changelog and fix typo

While not a particularly good idea, it's possible to use `object_id` as
an attribute name, typically by defining a polymorphic association named
`object`. Since 718a32ca, transactional
callbacks deduplicate records by their `object_id`, but this causes
incorrect behaviour when the record has an attribute with that name.

Using `__id__` instead makes a naming collision much less likely.

Follow up to 404e1a0a.

Co-Authored-By: NEileen M. Uchitelle <eileencodes@users.noreply.github.com>

This PR allows for passing `if_exists` options to the `remove_index`
method so that we can ignore already removed indexes. This work follows
column `if/if_not_exists` from #38352 and `:if_not_exists` on `add_index`
from #38555.

We've found this useful at GitHub, there are migrations where we don't
want to raise if an index was already removed. This will allow us to
remove a monkey patch on `remove_index`.

I considered raising after the `index_name_for_remove` method is called
but that method will raise if the index doesn't exist before we get to
execute. I have a commit that refactors this but after much
consideration this change is cleaner and more straightforward than other
ways of implementing this.

This change also adds a little extra validation to the `add_index` test.
Fix `nodoc` on edited methods.

#29589 changed merging scope order to allow to unscope default scopes on
association scope (#29611), but that caused a regression #38811 that
accidentally allow join constraint which is required.

```
% bin/test test/cases/associations/has_many_associations_test.rb -n test_unscopes_the_default_scope_of_associated_model_when_used_with_include
Run options: -n test_unscopes_the_default_scope_of_associated_model_when_used_with_include --seed 32978

# Running:

E

Error:
HasManyAssociationsTest#test_unscopes_the_default_scope_of_associated_model_when_used_with_include:
NoMethodError: undefined method `children' for nil:NilClass
    ~/rails/activerecord/lib/active_record/associations/join_dependency/join_association.rb:39:in `block in join_constraints'
    ~/rails/activerecord/lib/active_record/associations/join_dependency/join_association.rb:30:in `reverse_each'
    ~/rails/activerecord/lib/active_record/associations/join_dependency/join_association.rb:30:in `with_index'
    ~/rails/activerecord/lib/active_record/associations/join_dependency/join_association.rb:30:in `join_constraints'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:171:in `make_constraints'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:196:in `block in walk'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:196:in `each'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:196:in `flat_map'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:196:in `walk'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:90:in `block in join_constraints'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:87:in `each'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:87:in `flat_map'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:87:in `join_constraints'
    ~/rails/activerecord/lib/active_record/relation/query_methods.rb:1226:in `build_join_query'
    ~/rails/activerecord/lib/active_record/relation/query_methods.rb:1211:in `build_joins'
    ~/rails/activerecord/lib/active_record/relation/query_methods.rb:1091:in `build_arel'
    ~/rails/activerecord/lib/active_record/relation/query_methods.rb:1063:in `arel'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:419:in `block in limited_ids_for'
    ~/rails/activerecord/lib/active_record/relation.rb:867:in `skip_query_cache_if_necessary'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:419:in `limited_ids_for'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:398:in `apply_join_dependency'
    ~/rails/activerecord/lib/active_record/relation.rb:839:in `block in exec_queries'
    ~/rails/activerecord/lib/active_record/relation.rb:867:in `skip_query_cache_if_necessary'
    ~/rails/activerecord/lib/active_record/relation.rb:834:in `exec_queries'
    ~/rails/activerecord/lib/active_record/relation.rb:639:in `load'
    ~/rails/activerecord/lib/active_record/relation.rb:250:in `records'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:508:in `find_take'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:98:in `take'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:458:in `find_one'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:442:in `find_with_ids'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:69:in `find'
    ~/rails/activerecord/test/cases/associations/has_many_associations_test.rb:2689:in `block in <class:HasManyAssociationsTest>'

bin/test test/cases/associations/has_many_associations_test.rb:2683
```

Required join constraint should not be allowed to unscoping.

Fixes #38811.

This removes ibm_db, informix, mssql, oracle, and oracle12 Arel visitors
which are not used in the code base.

Actually oracle and oracle12 visitors are used at oracle-enhanced
adapter, but now I think that those visitors should be in the adapter's
repo like sqlserver adapter and the dedicated Arel visitor
(https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/blob/master/lib/arel/visitors/sqlserver.rb),
otherwise it is hard to find a bug and review PRs for the oracle
visitors (e.g. #35838, #37646), since we don't have knowledge and
environment enough for Oracle.

  * Use ASSETS_ROOT (defined in activerecord/test/config.rb) to guarantee a
    valid path to schema_dump_5_1.yml.

Previously, if `build_association` was called multiple times for a `has_one` association but never committed to the database, the first newly-associated record would trigger `touch` during the attempted removal of the record.

For example:

    class Post < ActiveRecord::Base
      has_one :comment, inverse_of: :post, dependent: :destroy
    end

    class Comment < ActiveRecord::Base
      belongs_to :post, inverse_of: :comment, touch: true
    end

    post = Post.create!
    comment_1 = post.build_comment
    comment_2 = post.build_comment

When `comment_2` is initialized, the `has_one` would attempt to destroy `comment_1`, triggering a `touch` on `post` from an association record that hasn't been committed to the database.

This removes the attempt to delete an associated `has_one` unless it’s persisted.

Followup to b94efe9f.

Example failure: https://buildkite.com/rails/rails/builds/68187#333e3624-ac0d-4b23-95b9-f068d9901093/1016-1028

These tests expect the 2nd Account fixture (with a NULL `firm_id`) to be
in the first four rows of the result set, but that behavior is not
guaranteed without an `order`.

FrontBase and IBM_DB adapters do not work for Rails 5.2.

https://rubygems.org/gems/ruby-frontbase
https://rubygems.org/gems/ibm_db

Related #15137.

Firebird related code is already removed in #15137.

We have two `current_adapter?(:DB2Adapter)` in tests, but the adapter is
no longer maintained (last release is November 15, 2012).

https://rubygems.org/gems/db2

Yet another (latest) DB2 adapter (`IBM_DBAdapter`) might support Rails
5.0.7, but apparently do not work for Rails 5.2.

https://rubygems.org/gems/ibm_db

We have few lines mention about DB2 in the doc, but now there is no
worth for almost all current users.

#37523 has a regression that ignore extra scoping in callbacks when
create on association relation.

It should respect `klass.current_scope` even when create on association
relation to allow extra scoping in callbacks.

Fixes #38741.

This module was added in 16ae3db5 to
allow `ActiveRecord::AttributeMethods::Dirty` to define callbacks and
still have its `_update_record` method wrapped by the version defined in
`ActiveRecord::Callbacks`, so that updates in `before_update` callbacks
are taken into account for partial writes.

The callbacks that created this circular dependency were removed in
34f075fe, so we can move the callback
definitions back to the `Callbacks` module.

Testing that when you set a symbol for an enum type, you get a string
back and that when you set a string you also get a string back.

* Fix EagerLoadPolyAssocsTest setup

  * EagerLoadPolyAssocsTest includes a Remembered module in multiple test
    ActiveRecord classes. The module is supposed to keep track of records
    created for each of the included classes individually, but it collects all
    records for every class. This happens because @@remembered is defined on the
    Remembered module and shared between the ActiveRecord classes.  This only
    becomes an issue for databases (like CockroachDB) that use random primary
    keys instead of sequential ones by default.
  * To fix the bug, we can make the remembered collection name unique per
    ActiveRecord class.

* Update EagerLoadPolyAssocsTest test setup

  * Instead of defining remembered as a class variable, we can define it as an
    instance variable that will be unique to every class that includes the
    Remembered module.

    - adds the option `metadata_table` to a database connection

When we try to create a table which already exists which also adds
indexes, then the `if_not_exists` option passed to `create_table` is
not extended to indexes. So the migration results into an error if the
table and indexes already exist.
This change extends the `if_not_exists` support to `add_index` so that
if the migration tries to create a table which also has existing
indexes, error won't be raised.
Also as a side-effect individual `add_index` calls will also accept
`if_not_exists` option and respect it henceforth.

[Prathamesh Sonpatki, Justin George]

https://buildkite.com/rails/rails/builds/67891#867b2766-7984-4280-90d6-7f0412e2d239/1015-1026

This reverts commit f265e0dd, reversing
changes made to 08dfa921.

Reverted due to surprising behavior for applications. We need to
deprecate this behavior first instead of raising by default.