Use whatever adapter-provided means we have available to ensure forked
children don't send quit/shutdown/goodbye messages to the server on
connections that belonged to their parent.

Add new error class `StatementTimeout` which will be raised when statement timeout exceeded (#31129)

We are sometimes using The MAX_EXECUTION_TIME hint for MySQL depending
on the situation. It will prevent catastrophic performance down by wrong
performing queries.

The new error class `StatementTimeout` will make to be easier to handle
that case.

https://dev.mysql.com/doc/refman/5.7/en/optimizer-hints.html#optimizer-hints-execution-time

Follow up of #30360.

since the minimum version of PostgreSQL currently Rails supports is 9.1,
there is no need to handle if `supports_extensions?`

Refer https://www.postgresql.org/docs/9.1/static/sql-createextension.html
"CREATE EXTENSION"

This basically reverts 9d4f79d3

We already found the longer sequence name, but we could not consider
whether it was the sequence name created by serial type due to missed a
max identifier length limitation. I've addressed the sequence name
consideration to respect the max identifier length.

Fixes #28332.

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.

`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.

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]]
```

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

`type_map` is an internal API and it is only used in the connection
adapter. And also, some type map initializer methods requires passed
`type_map`, but those instances already has `type_map` in itself.
So we don't need explicit passing `type_map` to the initializers.

connection_active? will sometimes return true when the connection is actually dead/disconnected (see #3392 for a discussion of why this is).  When this happens, a query is run on the dead connection which causes various postgres connection errors to be raised.  This fix catches any such errors and ignores them.

Closes #29760

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.

Because identifiers in SQL could include a single quote.

Related #24950, #26784.

`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

Revert "Merge pull request #27636 from mtsmfm/disable-referential-integrity-without-superuser-privilege-take-2"

This reverts commit c1faca63, reversing
changes made to 8c658a0e.

See https://github.com/rails/rails/pull/27636#issuecomment-297534129

Address #28797

In the previous versions of PostgreSQL, `CURRENT_DATE` converted to `('now'::text)::date`
and `CURRENT_TIMESTAMP` converted to `now()`.

Refer these discussions and commit at PostgreSQL :
https://www.postgresql.org/message-id/flat/5878.1463098164%40sss.pgh.pa.us#5878.1463098164@sss.pgh.pa.us
https://github.com/postgres/postgres/commit/0bb51aa96783e8a6c473c2b5e3725e23e95db834

Transactional-fixture using tests with racing threads and inter-thread
synchronisation inside transaction blocks will now deadlock... but
without this, they would just crash.

In 5.0, the threads didn't share a connection at all, so it would've
worked... but with the main thread inside the fixture transaction, they
wouldn't've been able to see each other.

So: as far as I can tell, the set of operations this "breaks" never had
a compelling use case. Meanwhile, it provides an increased level of
coherency to the operational feel of transactional fixtures.

If this does cause anyone problems, they're probably best off disabling
transactional fixtures on the affected tests, and managing transactions
themselves.

Re-create https://github.com/rails/rails/pull/21233

eeac6151 was reverted (127509c0) because it breaks tests.

----------------

ref: 72c15572

- We must use `authors` fixture with `author_addresses` because of its foreign key constraint.
- Tests require PostgreSQL >= 9.4.2 because it had a bug about `ALTER CONSTRAINTS` and fixed in 9.4.2.

Because this comment is not document for `supports_ranges?`

ref: https://github.com/rails/rails/pull/27636#discussion_r107560081

The old top level classes PGconn, PGresult and PGError were deprecated
since pg-0.13.0: https://github.com/ged/ruby-pg/blob/master/History.rdoc#v0130-2012-02-09-michael-granger-gedfaeriemudorg

This reverts commit 192db644, reversing
changes made to 98939553.

We can't sacrifice correctness for performance.

Use “distinct” to filter redundant types from pg_typeto reduce memory bloat especially in multi-schema structure database

Actually `index_name_length` depend on `max_identifier_length`, not
always 63.

`supports_migrations?` was added at 4160b518 to determine if schema
statements (`create_table`, `drop_table`, etc) are implemented in the
adapter. But all tested databases has been supported migrations since
a4fc93c3 at least.

`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

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 ]

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

Closes #27980

Closes #27979