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CREATE TABLE watchlist (
  wl_user int(5) unsigned NOT NULL DEFAULT '0',
  wl_namespace int(11) NOT NULL DEFAULT '0',
  wl_title varbinary(255) NOT NULL DEFAULT '',
  wl_notificationtimestamp varbinary(14) DEFAULT NULL,
  UNIQUE KEY wl_user (wl_user,wl_namespace,wl_title),
  KEY namespace_title (wl_namespace,wl_title)
) ENGINE=InnoDB DEFAULT CHARSET=binary;

InnoDB uses the unique key wl_user as a primary key. Since PK is clustered, secondary indexes contain hidden copies of the PK columns, potentially making them quite large.

Bug 66089 adds another secondary index, wl_user_notificationtimestamp, so it's time to consider adding a surrogate primary key wl_id.