Add support for virtual tables (SQL Views) to the DB Schema Definition API

Problem/Motivation

The Database Schema API supports defining and managing database tables, but it has no equivalent support for SQL views.

Modules that need an SQL view must currently execute DDL directly and manage database portability, lifecycle, dependencies, and test cleanup themselves.

Core would benefit from this with at least a couple of use cases:

• Deprecating/renaming database tables. For example, Workspaces would like to rename the workspace_association table to workspace_tracker. BC could be provided for this rename by creating a virtual workspace_association table that matches the data of the new one.
• After #3343634: Add "json" as core data type to Schema and Database API is done, we can extract data from JSON columns into table structures, for example to match the structure of dedicated entity field tables.

Proposed resolution

Extend the Database Schema API so modules can define, create, discover, replace, and remove SQL views in a database-independent way.

Modules can declare views alongside tables in hook_schema(). Tables are created before their views and views are removed before their underlying tables.

Example:

$views[] = new View(
  name: 'example_active_items',
  query: new ViewQuery(
    'SELECT [id], [label] FROM {example_item} WHERE [active] = 1'
  ),
  description: 'Active example items.',
);

return new Schema(
  type: SchemaDefinitionType::Module,
  name: 'example',
  tables: $tables,
  views: $views,
);

API changes

New schema definition classes:

• Drupal\Core\Database\SchemaDefinition\View
• Drupal\Core\Database\SchemaDefinition\ViewQuery

New methods on Drupal\Core\Database\Schema:

public function createView(
  string $name,
  string $query,
  bool $replace = FALSE,
): void;

public function createViewFromDefinition(View $view): void;

public function dropView(string $name): bool;

public function dropViews(array $views): void;

public function viewExists(
  string $name,
  bool $add_prefix = TRUE,
): bool;

public function findViews(string $view_expression): array;

SchemaDefinition\Schema receives a new optional views constructor argument and provides:

• viewNames()
• getViewDefinition()

Schema definitions containing views cannot be converted to the legacy array representation because that format has no representation for SQL views.

Data model changes

Modules can create SQL views alongside their tables. No existing table schemas or stored data are changed.

User interface changes

None.

Remaining tasks

• Review the new API and database-driver implementations.

Release notes snippet

TBD.