How to monitor the performance of a mysql database during development? Is there a standard approach to monitor the performance of an application and its database, which is still in design? 

What I'm doing so far: I develop queries and tests directly in mysql workbench and monitor the msec used per query. On top of that I'm using mocha tests in node.js to test the database api and also try here to measure the typical impact on the database.

The standard approach I know is: do your design and one day, as the amount of data has grown, there is a bad surprise, that the database needs to be optimized. I'm asking for good practices to monitor performance from the beginning.
My favorite monitoring tool is the slowlog, with long_query_time=1.  And use pt-query-digest to summarize the results.  Then work on improving the first few queries in the list.  They will, by default, be ordered with the "worst" queries first -- either slow but infrequent or fast but frequent (or both).

Digging into why a particular query is slow (or frequent) may lead to a missing index, the need for a 'composite' index, reformulation of a query, redesign of the code, tuning, more RAM, or any of a dozen other things.

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