[Meta] Remove database specific logic from core

Problem/Motivation

Drupal core has still lots of database specific logic that has not been moved to DTBNG. There are many parts of core that take conditional actions depending on which database backend is being used:

$db_type = Database::getConnection()->databaseType();
    if (in_array($db_type, ['mysql', 'pgsql'])) {
      $offset = '+00:00';
      static $already_set = FALSE;
      if (!$already_set) {
        if ($db_type == 'pgsql') {
          Database::getConnection()->query("SET TIME ZONE INTERVAL '$offset' HOUR TO MINUTE");
        }
        elseif ($db_type == 'mysql') {
          Database::getConnection()->query("SET @@session.time_zone = '$offset'");
        }

        $already_set = TRUE;
      }
    }

This prevents contributed database drivers from being able to fully support Drupal, and completely defeats the purpose of having a Database Abstraction layer.

How much code is affected? Doing a quick find-in-files of "databaseType()":

...\lib\Drupal\Core\Command\DbDumpCommand.php(138): if ($connection->databaseType() !== 'mysql') {
...\modules\path\tests\src\Functional\PathAliasTest.php(103): if ($connection->databaseType() != 'sqlite') {
...\modules\path\tests\src\Functional\PathAliasTest.php(257): if ($connection->databaseType() != 'sqlite') {
...\modules\simpletest\src\Tests\KernelTestBaseTest.php(345): if ($connection->databaseType() != 'sqlite') {
...\modules\system\src\Tests\Entity\Update\LangcodeToAsciiUpdateTest.php(30): if (Database::getConnection()->databaseType() !== 'mysql') {
...\modules\system\tests\src\Kernel\Scripts\DbDumpCommandTest.php(29): if (Database::getConnection()->databaseType() !== 'mysql') {
...\modules\views\src\Plugin\views\argument\StringArgument.php(183): if (Database::getConnection()->databaseType() == 'sqlite') {
...\modules\views\src\Plugin\views\argument\StringArgument.php(189): if (Database::getConnection()->databaseType() == 'pgsql') {
...\modules\views\src\Plugin\views\argument\StringArgument.php(215): if ($this->options['case'] != 'none' && Database::getConnection()->databaseType() == 'pgsql') {
...\modules\views\src\Plugin\views\argument\StringArgument.php(284): if ($this->options['case'] != 'none' && Database::getConnection()->databaseType() == 'pgsql') {
...\modules\views\src\Plugin\views\query\Sql.php(1758): $db_type = Database::getConnection()->databaseType();
...\modules\views\src\Plugin\views\query\Sql.php(1796): $db_type = Database::getConnection()->databaseType();
...\modules\views\src\Plugin\views\query\Sql.php(1819): $db_type = Database::getConnection()->databaseType();
...\tests\Drupal\KernelTests\Core\Command\DbDumpTest.php(83): $this->skipTests = Database::getConnection()->databaseType() !== 'mysql';
...\tests\Drupal\KernelTests\Core\Database\ConnectionTest.php(125): if (Database::getConnection()->databaseType() !== 'mysql' || !defined('\PDO::MYSQL_ATTR_MULTI_STATEMENTS')) {
...\tests\Drupal\KernelTests\Core\Database\ConnectionTest.php(162): if (Database::getConnection()->databaseType() !== 'pgsql') {
...\tests\Drupal\KernelTests\Core\Database\SchemaTest.php(66): if (Database::getConnection()->databaseType() == 'mysql') {
...\tests\Drupal\KernelTests\Core\Database\SchemaTest.php(189): $db_type = Database::getConnection()->databaseType();
...\tests\Drupal\KernelTests\Core\Database\SchemaTest.php(266): if (Database::getConnection()->databaseType() != 'mysql') {
...\tests\Drupal\KernelTests\Core\Database\SchemaTest.php(430): if (Database::getConnection()->databaseType() == 'mysql') {
...\tests\Drupal\KernelTests\Core\Database\SchemaTest.php(838): $db_type = Database::getConnection()->databaseType();
...\tests\Drupal\KernelTests\Core\Database\SelectComplexTest.php(236): $db_type = Database::getConnection()->databaseType();
...\tests\Drupal\KernelTests\KernelTestBaseTest.php(226): if ($connection->databaseType() != 'sqlite') {
...\tests\Drupal\Tests\Core\Database\Stub\Connection.php(47): public function databaseType() {

Proposed resolution

Remove any code in core that is in the form of:

   $db_type = Database::getConnection()->databaseType();
    if ($db_type == 'whatever') {
      // I'm not portable.
   }

Remaining tasks

Do it.

Child issues:

• #2657888: Add Date function support in DBTNG
• #2881522: Add a Schema::findPrimaryKeyColumns method to remove database specific logic from test

User interface changes

API changes

Data model changes