ActiveRecord call ConnectionPool#checkout and create new connection to
MySQL when receive a first HTTP request.
At this time, register to connection pool its connection in
but, Rack::Handler::WEBrick#service call body.close in ensure section
when return the HTTP response.
It call another method. after some call, it call
ConnectionPool#clear_reloadable_connections and clear its connection
As a result, the connection was created by every request and spend
time for connection.
How to use connection pool?
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