Slow query - participant report

While writing a custom participant report I noticed the built in one was slow. On looking at the query I noticed the join in the report doesn't match the index - resulting in an unindexed join

Altering the join from
LEFT JOIN civicrm_line_item line_item_civireport

AND line_item_civireport.entity_id = {$this->_aliases['civicrm_participant']}.id
";

to
LEFT JOIN civicrm_line_item line_item_civireport
ON line_item_civireport.entity_table = 'civicrm_participant'
AND line_item_civireport.entity_id = {$this->_aliases['civicrm_participant']}.id
";

fixes this query - although I suspect the join should not have actually been called - in this instance - but I am not investigating that.