initializeDataTable example

// Prepare basic information about the entity type.       /** @var \Drupal\Core\Entity\Sql\DefaultTableMapping $table_mapping */
      $table_mapping = $this->getTableMapping($entity_type, $this->fieldStorageDefinitions);
      $tables = $this->getEntitySchemaTables($table_mapping);

      // Initialize the table schema.       $schema[$tables['base_table']] = $this->initializeBaseTable($entity_type);
      if (isset($tables['revision_table'])) {
        $schema[$tables['revision_table']] = $this->initializeRevisionTable($entity_type);
      }
      if (isset($tables['data_table'])) {
        $schema[$tables['data_table']] = $this->initializeDataTable($entity_type);
      }
      if (isset($tables['revision_data_table'])) {
        $schema[$tables['revision_data_table']] = $this->initializeRevisionDataTable($entity_type);
      }

      // We need to act only on shared entity schema tables.       $table_names = array_diff($table_mapping->getTableNames(), $table_mapping->getDedicatedTableNames());
      foreach ($table_names as $table_name) {
        if (!isset($schema[$table_name])) {
          $schema[$table_name] = [];
        }