processIdentifierSchema example

$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     // tables.     if (($table_name === $base_table && $field_name === $this->entityType->getKey('id')) ||
      ($table_name === $revision_table && $field_name === $this->entityType->getKey('revision'))) {
      $this->processIdentifierSchema($schema, $field_name);
    }

    return $schema;
  }

  /** * Adds an index for the specified field to the given schema definition. * * @param \Drupal\Core\Field\FieldStorageDefinitionInterface $storage_definition * The storage definition of the field for which an index should be added. * @param array $schema * A reference to the schema array to be updated. * @param bool $not_null * (optional) Whether to also add a 'not null' constraint to the column * being indexed. Doing so improves index performance. Defaults to FALSE, * in case the field needs to support NULL values. * @param int $size * (optional) The index size. Defaults to no limit. */