Performance Implications when Views JOIN added twice? I have a view with a flag (relationship, only show flagged nodes). I have modified it a few times and now I see in the query output that views has the join added twice:

INNER JOIN {flag_content} flag_content_node ON node.nid = flag_content_node.content_id AND flag_content_node.fid = '9'
LEFT JOIN {flag_content} flag_content ON node.nid = flag_content.content_id AND flag_content.fid = '9'


Is there a performance hit for the extra LEFT JOIN?
Astonish Design did a good write up on this topic: Writing Efficient Queries in Drupal.

You should measure your specific queries though, it always depends on the amount of data you are working with, proper indices, mysql configuration and so on. Joining too many tables will almost always increase your query execution time, keep that in mind.