Properly fix table schema adding constraint (MySQL and PostgreSQL working version)

drupal-fix-table.php

/**
 * This function is reentrant.
 */
function mymodule_install_schema_fix() {
  $constraints = [
    'lo_register_phone' => [
      'fk_lo_register_phone_user' => "ALTER TABLE {lo_register_phone} ADD CONSTRAINT {fk_lo_register_phone_user} FOREIGN KEY (uid) REFERENCES {users} (uid) ON DELETE CASCADE",
    ],
  ];
  foreach ($constraints as $table => $statements) {
    if (db_table_exists($table)) {
      foreach ($statements as $statement) {
        try {
          db_query($statement);
        } catch (\PDOException $e) {
          switch ($e->getCode()) {
            case 42710: // PostgreSQL constraint already exists
              continue;
            case 23000: // MySQL duplicate key in table
              continue;
            default:
              throw $e;
          }
        }
      }
    }
  }
}