drupal/core/modules/migrate_drupal/migrate_drupal.install

<?php

/**
 * @file
 * Contains install and update functions for Migrate Drupal
 */

use Drupal\Core\Database\Database;

/**
 * Implements hook_update_last_removed().
 */
function migrate_drupal_update_last_removed() {
  return 8601;
}

/**
 * Add revision ID to entity reference translation migrate_map tables..
 */
function migrate_drupal_update_8901(&$sandbox) {
  $schema = Database::getConnection()->schema();
  $table_expression = 'migrate_map%entity_reference_translation%node%';
  $tables = $schema->findTables($table_expression);
  foreach ($tables as $table) {
    // Move language code to sourceid3.
    $spec = [
      'type' => 'varchar',
      'length' => 12,
      'not null' => TRUE,
    ];
    $schema->changeField($table, 'sourceid2', 'sourceid3', $spec);

    // Add revision ID.
    $spec = [
      'type' => 'int',
      'unsigned' => TRUE,
      'not null' => TRUE,
      'default' => 0,
    ];
    $schema->addField($table, 'sourceid2', $spec);

    // Add sourceid2 to index.
    $spec = [
      'fields' => [
        'sourceid1' => [
          'type' => 'int',
          'not_null' => TRUE,
        ],
        'sourceid2' => [
          'type' => 'int',
          'not_null' => TRUE,
        ],
        'sourceid3' => [
          'type' => 'varchar',
          'length' => 12,
          'not null' => TRUE,
        ],
      ],
    ];
    $fields = [
      'sourceid1',
      'sourceid2',
      'sourceid3',
    ];
    $schema->dropIndex($table, 'source');
    $schema->addIndex($table, 'source', $fields, $spec);
  }
}