Create a SQL index on History.url.

This will hopefully speed up historyContains but does not seem to speed
up the completion query, unfortunately.

qutebrowser/browser/history.py

@@ -78,6 +78,7 @@ class WebHistory(sql.SqlTable):
     def __init__(self, parent=None):
         super().__init__("History", ['url', 'title', 'atime', 'redirect'],
                          parent=parent)
+        self.create_index('HistoryIndex', 'url', where='not redirect')
         self._between_query = sql.Query('SELECT * FROM History '
                                         'where not redirect '
                                         'and not url like "qute://%" '

qutebrowser/misc/sql.py

@@ -122,6 +122,18 @@ class SqlTable(QObject):
         # pylint: disable=invalid-name
         self.Entry = collections.namedtuple(name + '_Entry', fields)
 
+    def create_index(self, name, field, where):
+        """Create an index over this table.
+
+        Args:
+            name: Name of the index, should be unique.
+            field: Name of the field to index.
+            where: WHERE clause for a partial index.
+        """
+        q = Query("CREATE INDEX IF NOT EXISTS {} ON {} ({}) WHERE {}"
+                  .format(name, self._name, field, where))
+        q.run()
+
     def __iter__(self):
         """Iterate rows in the table."""
         q = Query("SELECT * FROM {}".format(self._name))