addBaseFieldIndex example

    // creates its schema.     $this->addBaseField();
    $this->assertTrue($this->entityDefinitionUpdateManager->needsUpdates(), 'EntityDefinitionUpdateManager reports that updates are needed.');
    $changes = $this->entityDefinitionUpdateManager->getChangeSummary();
    $this->assertCount(1, $changes['entity_test_update']);
    $this->assertEquals('The A new base field field needs to be installed.', strip_tags($changes['entity_test_update'][0]));
    $this->applyEntityUpdates();
    $this->assertTrue($this->database->schema()->fieldExists('entity_test_update', 'new_base_field'), 'Column created in shared table for new_base_field.');

    // Add an index on the base field, ensure the update manager reports it,     // and the update creates it.     $this->addBaseFieldIndex();
    $this->assertTrue($this->entityDefinitionUpdateManager->needsUpdates(), 'EntityDefinitionUpdateManager reports that updates are needed.');
    $changes = $this->entityDefinitionUpdateManager->getChangeSummary();
    $this->assertCount(1, $changes['entity_test_update']);
    $this->assertEquals('The A new base field field needs to be updated.', strip_tags($changes['entity_test_update'][0]));
    $this->applyEntityUpdates();
    $this->assertTrue($this->database->schema()->indexExists('entity_test_update', 'entity_test_update_field__new_base_field'), 'Index created.');

    // Remove the above index, ensure the update manager reports it, and the     // update deletes it.     $this->removeBaseFieldIndex();
    $this->assertTrue($this->entityDefinitionUpdateManager->needsUpdates(), 'EntityDefinitionUpdateManager reports that updates are needed.');