SqlContentEntityStorageSchema::getFieldSchemaData

protected SqlContentEntityStorageSchema::getFieldSchemaData($field_name, array $field_schema, array $column_mapping, $schema_key)

protected function getFieldSchemaData($field_name, array $field_schema, array $column_mapping, $schema_key) {
  $data = array();

  foreach ($field_schema[$schema_key] as $key => $columns) {
    // To avoid clashes with entity-level indexes or unique keys we use
    // "{$entity_type_id}_field__" as a prefix instead of just
    // "{$entity_type_id}__". We additionally namespace the specifier by the
    // field name to avoid clashes when multiple fields of the same type are
    // added to an entity type.
    $entity_type_id = $this->entityType->id();
    $real_key = $this->getFieldSchemaIdentifierName($entity_type_id, $field_name, $key);
    foreach ($columns as $column) {
      // Allow for indexes and unique keys to specified as an array of column
      // name and length.
      if (is_array($column)) {
        list($column_name, $length) = $column;
        $data[$real_key][] = array($column_mapping[$column_name], $length);
      }
      else {
        $data[$real_key][] = $column_mapping[$column];
      }
    }
  }

  return $data;
}