"SHOW COLUMNS" request called by trackers (no replies)

Hello,

I'm working with Google Cloud, 3 Virtual Machines as trackers, working with Google Cloud SQL.

I'm encountering an issue which prevent me completely to use Piwik : my trackers are calling the requests "SHOW COLUMNS FROM piwik_log_visit / piwik_log_link_visit_action [...]"
I absolutely don't know why this request is called, but Google Cloud SQL seems to thread this query, and by threading it with the "one_thread_per_connection" (thread_handling parameter in SQL), it grows to the max_connections instantly => Every next requests encounter the "Too many connection" error from MySQL..
Usually, it doesn't last more than 30 minutes (which is HUGE amount of data lost), but today it's being executed for more than 4 hours, and it does not finish at all. My database contains about 29G of data, and I'm tracking more than 500k visits per day, so maybe that's why it takes so long, but it doesn't explain why this request is called..

I can't track anything so I'm losing all of my data atm.. I searched a lot, even posted on stackoverflow but I got no answer for the moment (http://stackoverflow.com/questions/24631874/too-many-connections-mysql-threads-reaching-max-connections-with-one-thread-pe)

Is there a solution?

Thanks

Stephane