BlockContentStorageSchema.php

<?php

namespace Drupal\block_content;

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

/**
 * Defines the block content schema handler.
 */
class BlockContentStorageSchema extends SqlContentEntityStorageSchema {
  
  /**
   * {@inheritdoc}
   */
  protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $storage_definition, $table_name, array $column_mapping) : array {
    $schema = parent::getSharedTableFieldSchema($storage_definition, $table_name, $column_mapping);
    $field_name = $storage_definition->getName();
    if ($table_name === $this->storage
      ->getDataTable() && $field_name === 'reusable') {
      $this->addSharedTableFieldIndex($storage_definition, $schema);
    }
    return $schema;
  }

}