getFieldIndexes example

// Use the initial value of the field storage, if available.       if ($initial_value && isset($initial_value[$field_column_name])) {
        $schema['fields'][$schema_field_name]['initial'] = SqlContentEntityStorageSchema::castValue($column_schema, $initial_value[$field_column_name]);
      }
      if (!empty($initial_value_from_field)) {
        $schema['fields'][$schema_field_name]['initial_from_field'] = $initial_value_from_field[$field_column_name];
      }
    }

    if (!empty($field_schema['indexes'])) {
      $schema['indexes'] = $this->getFieldIndexes($field_name, $field_schema, $column_mapping);
    }

    if (!empty($field_schema['unique keys'])) {
      $schema['unique keys'] = $this->getFieldUniqueKeys($field_name, $field_schema, $column_mapping);
    }

    if (!empty($field_schema['foreign keys'])) {
      $schema['foreign keys'] = $this->getFieldForeignKeys($field_name, $field_schema, $column_mapping);
    }

    // Process the 'id' and 'revision' entity keys for the base and revision