ItemStorageSchema.php

<?php

namespace Drupal\aggregator;

use Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema;
use Drupal\Core\Field\FieldStorageDefinitionInterface;

/**
 * Defines the item schema handler.
 */
class ItemStorageSchema extends SqlContentEntityStorageSchema {
    
    /**
     * {@inheritdoc}
     */
    protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $storage_definition, $table_name, array $column_mapping) {
        $schema = parent::getSharedTableFieldSchema($storage_definition, $table_name, $column_mapping);
        $field_name = $storage_definition->getName();
        if ($table_name == $this->storage
            ->getBaseTable()) {
            switch ($field_name) {
                case 'timestamp':
                    $this->addSharedTableFieldIndex($storage_definition, $schema, TRUE);
                    break;
                case 'fid':
                    $this->addSharedTableFieldForeignKey($storage_definition, $schema, 'aggregator_feed', 'fid');
                    break;
            }
        }
        return $schema;
    }

}