Before:

```
   (16.4ms)  TRUNCATE TABLE `author_addresses`
   (20.5ms)  TRUNCATE TABLE `authors`
   (19.4ms)  TRUNCATE TABLE `posts`
```

After:

```
  Truncate Tables (19.5ms)  TRUNCATE TABLE `author_addresses`;
TRUNCATE TABLE `authors`;
TRUNCATE TABLE `posts`
```

This is to easier make `truncate_tables` to bulk statements.

Adds a method to ActiveRecord allowing records to be inserted in bulk without instantiating ActiveRecord models. This method supports options for handling uniqueness violations by skipping duplicate records or overwriting them in an UPSERT operation.

ActiveRecord already supports bulk-update and bulk-destroy actions that execute SQL UPDATE and DELETE commands directly. It also supports bulk-read actions through `pluck`. It makes sense for it also to support bulk-creation.

Related cbcdecd2, 2a56b2d9.

This is a regression caused by cbcdecd2.

If query caching is enabled, prepared statement handles are never
re-used, since we missed that a query is preprocessed when query caching
is enabled, but doesn't keep the `preparable` flag.

We should care about that case.

Since #26002, `id_value` is no longer passed to `sql_for_insert`.

This PR adds the ability to prevent writes to a database even if the
database user is able to write (ie the database is a primary and not a
replica).

This is useful for a few reasons: 1) when converting your database from
a single db to a primary/replica setup - you can fix all the writes on
reads early on, 2) when we implement automatic database switching or
when an app is manually switching connections this feature can be used
to ensure reads are reading and writes are writing. We want to make sure
we raise if we ever try to write in read mode, regardless of database
type and 3) for local development if you don't want to set up multiple
databases but do want to support rw/ro queries.

This should be used in conjunction with `connected_to` in write mode.
For example:

```
ActiveRecord::Base.connected_to(role: :writing) do
  Dog.connection.while_preventing_writes do
    Dog.create! # will raise because we're preventing writes
  end
end

ActiveRecord::Base.connected_to(role: :reading) do
  Dog.connection.while_preventing_writes do
    Dog.first # will not raise because we're not writing
  end
end
```

In Ruby 2.3 or later, `String#+@` is available and `+@` is faster than `dup`.

```ruby
# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report('+@') { +"" }
  x.report('dup') { "".dup }
  x.compare!
end
```

```
$ ruby -v benchmark.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
Warming up --------------------------------------
                  +@   282.289k i/100ms
                 dup   187.638k i/100ms
Calculating -------------------------------------
                  +@      6.775M (± 3.6%) i/s -     33.875M in   5.006253s
                 dup      3.320M (± 2.2%) i/s -     16.700M in   5.032125s

Comparison:
                  +@:  6775299.3 i/s
                 dup:  3320400.7 i/s - 2.04x  slower

```

This is a follow up and/or an alternative of #33844.

Unlike #33844, this would attempt to construct unprepared statement only
when bind params limit (mysql2 65535, pg 65535, sqlite3 249999) is
exceeded.

I only defined 65535 as the limit, not defined 249999 for sqlite3, since
it is an edge case, I'm not excited to add less worth extra code.

If a transaction is opened and closed without any queries being run, we
can safely omit the `BEGIN` and `COMMIT` statements, as they only exist
to modify the connection's behaviour inside the transaction. This
removes the overhead of those statements when saving a record with no
changes, which makes workarounds like `save if changed?` unnecessary.

This implementation buffers transactions inside the transaction manager
and materializes them the next time the connection is used. For this to
work, the adapter needs to guard all connection use with a call to
`materialize_transactions`. Because of this, adapters must opt in to get
this new behaviour by implementing `supports_lazy_transactions?`.

If `raw_connection` is used to get a reference to the underlying
database connection, the behaviour is disabled and transactions are
opened eagerly, as we can't know how the connection will be used.
However when the connection is checked back into the pool, we can assume
that the application won't use the reference again and reenable lazy
transactions. This prevents a single `raw_connection` call from
disabling lazy transactions for the lifetime of the connection.

to support Oracle database support identity data type

Oracle database does not support `INSERT .. DEFAULT VALUES`
then every insert statement needs at least one column name specified.

When `prefetch_primary_key?` returns `true` insert statement
always have the primary key name since the primary key value is selected
from the associated sequence. However, supporting identity data type
will make `prefetch_primary_key?` returns `false`
then no primary key column name added.
As a result, `empty_insert_statement_value` raises `NotImplementedError`

To address this error `empty_insert_statement_value` can take
one argument `primary_key` to generate insert statement like this.

`INSERT INTO "POSTS" ("ID") VALUES(DEFAULT)`

It needs arity change for the public method but no actual behavior changes for the bundled adapters.

Oracle enhanced adapter `empty_insert_statement_value` implementation will be like this:

```
def empty_insert_statement_value(primary_key)
  raise NotImplementedError unless primary_key
  "(#{quote_column_name(primary_key)}) VALUES(DEFAULT)"
end
```

[Raise NotImplementedError when using empty_insert_statement_value with Oracle](https://github.com/rails/rails/pull/28029)
[Add support for INSERT .. DEFAULT VALUES](https://community.oracle.com/ideas/13845)

Since #29504, mysql2 adapter lost ability to insert zero value on
primary key due to enforce `NO_AUTO_VALUE_ON_ZERO` disabled.
That is for using `DEFAULT` on auto increment column, but we can use
`NULL` instead in that case.

- The `insert_fixtures` method can be optimized by making a single multi statement query for all fixtures having the same connection instead of doing a single query per table
  - The previous code was bulk inserting fixtures for a single table, making X query for X fixture files
  - This patch builds a single **multi statement query** for every tables. Given a set of 3 fixtures (authors, dogs, computers):
  ```ruby
    # before
    %w(authors dogs computers).each do |table|
      sql = build_sql(table)
      connection.query(sql)
    end

    # after

    sql = build_sql(authors, dogs, computers)
    connection.query(sql)
  ```
- `insert_fixtures` is now deprecated, `insert_fixtures_set` is the new way to go with performance improvement
- My tests were done with an app having more than 700 fixtures, the time it takes to insert all of them was around 15s. Using a single multi statement query, it took on average of 8 seconds
- In order for a multi statement to be executed, mysql needs to be connected with the `MULTI_STATEMENTS` [flag](https://dev.mysql.com/doc/refman/5.7/en/c-api-multiple-queries.html), which is done before inserting the fixtures by reconnecting to da the database with the flag declared. Reconnecting to the database creates some caveats:
  1. We loose all open transactions; Inside the original code, when inserting fixtures, a transaction is open. Multple delete statements are [executed](https://github.com/rails/rails/blob/a681eaf22955734c142609961a6d71746cfa0583/activerecord/lib/active_record/fixtures.rb#L566) and finally the fixtures are inserted. The problem with this patch is that we need to open the transaction only after we reconnect to the DB otherwise reconnecting drops the open transaction which doesn't commit all delete statements and inserting fixtures doesn't work since we duplicated them (Primary key duplicate exception)...
    - In order to fix this problem, the transaction is now open directly inside the `insert_fixtures` method, right after we reconnect to the db
    - As an effect, since the transaction is open inside the `insert_fixtures` method, the DELETE statements need to be executed here since the transaction is open later
  2. The same problem happens for the `disable_referential_integrity` since we reconnect, the `FOREIGN_KEY_CHECKS` is reset to the original value
    - Same solution as 1. , the disable_referential_integrity can be called after we reconnect to the transaction
  3. When the multi statement query is executed, no other queries can be performed until we paginate over the set of results, otherwise mysql throws a "Commands out of sync" [Ref](https://dev.mysql.com/doc/refman/5.7/en/commands-out-of-sync.html)
    - Iterating over the set of results until `mysql_client.next_result` is false. [Ref](https://github.com/brianmario/mysql2#multiple-result-sets)
- Removed the `active_record.sql "Fixture delete"` notification, the delete statements are now inside the INSERT's one
- On mysql the `max_allowed_packet` is looked up:
  1. Before executing the multi-statements query, we check the packet length of each statements, if the packet is bigger than the max_allowed_packet config, an `ActiveRecordError` is raised
  2. Otherwise we concatenate the current sql statement into the previous and so on until the packet is `< max_allowed_packet`

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.

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

It seems that it accepts only HTTPS connections.
Ref: https://github.com/postgres/postgres/commit/7f77cbd996855a06fb742ea11adbe55c42b48fe2

This also reverts the change to enable prepared statements by default on
MySQL (though I suspect we could enable them and it'd be great). This
change brings back a collector closer to the old `Bind` collector in
Arel. However, this one lives in AR, since this is an AR specific need.
Additionally, we only use it for statement caching, since the new
substitute collector in Arel is higher performance for most cases.

A common source of bugs and code bloat within Active Record has been the
need for us to maintain the list of bind values separately from the AST
they're associated with. This makes any sort of AST manipulation
incredibly difficult, as any time we want to potentially insert or
remove an AST node, we need to traverse the entire tree to find where
the associated bind parameters are.

With this change, the bind parameters now live on the AST directly.
Active Record does not need to know or care about them until the final
AST traversal for SQL construction. Rather than returning just the SQL,
the Arel collector will now return both the SQL and the bind parameters.
At this point the connection adapter will have all the values that it
had before.

A bit of this code is janky and something I'd like to refactor later. In
particular, I don't like how we're handling associations in the
predicate builder, the special casing of `StatementCache::Substitute` in
`QueryAttribute`, or generally how we're handling bind value replacement
in the statement cache when prepared statements are disabled.

This also mostly reverts #26378, as it moved all the code into a
location that I wanted to delete.

/cc @metaskills @yahonda, this change will affect the adapters

Fixes #29766.
Fixes #29804.
Fixes #26541.
Close #28539.
Close #24769.
Close #26468.
Close #26202.

There are probably other issues/PRs that can be closed because of this
commit, but that's all I could find on the first few pages.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

`test_middleware_caches` is sometimes failed since #29454.
The failure is due to schema statements are affected by query caching.
Bypassing query caching for schema statements to avoid the issue.

Scalar values like arrays and hashes can't be inserted
directly into table. Previously, the way to determine if
the value is scalar was to try quoting it. If `quote` raised
with an error than the value has to be converted to YAML.

This flow is not very obvious. Ideally we could have
a `quotable?` method in the connection, but I think
that we can avoid begin/rescue block by simply checking
if the value is Array or Hash.

https://github.com/rails/rails/commit/aa31d21f5f4fc4d679e74a60f9df9706da7de373

Improves the performance from O(n) to O(1).
Previously it would require 50 queries to
insert 50 fixtures. Now it takes only one query.

Disabled on sqlite which doesn't support multiple inserts.

`supports_statement_cache?` was introduced in 3.1.0.beta1 (104d0b26) for
bind parameter substitution, but it is no longer used in 3.1.0.rc1
(73ff6797). Originally it should respect `prepared_statements` rather
than `supports_statement_cache?` (fd398475).
One more thing, named `supports_statement_cache?` is pretty misreading.
We have `StatementCache` and `StatementPool`. However,
`supports_statement_cache?` doesn't mean `StatementCache`, but
`StatementPool` unlike its name.

https://github.com/rails/rails/blob/v5.1.0/activerecord/lib/active_record/statement_cache.rb
https://github.com/rails/rails/blob/v5.1.0/activerecord/lib/active_record/connection_adapters/statement_pool.rb

Related #22973, #24708.

`select_all`, `select_one`, `select_value`, and `select_values` method
signature is `(arel, name = nil, binds = [])`.
But `select_rows` is `(sql, name = nil, binds = [])`.

Some methods were added to public API in
5b14129d and they should be not part of
the public API.

make sql statements frozen

dup if arel is not our string

expect runtime error

dont wrap runtime error in invalid

log errors will now be treated as runtime errors

update changelog