SQL query on /tasks/completed endpoint can be very slow

[djanderson]

Django ORM queries were written naively for speed of development. At this point, it's probably worth spending the time to determine hot queries and tweaking them.

One of the worst offenders is /tasks/completed/{schedule_entry_name} when the entry holds a lot of acquisitions, for example a stepped_freq_tdomain_iq.

To test,

• create 1 stepped_freq_tdomain_iq acquisition,
• log into the browsable api root with debugging enabled (currently on by default, see docker-compose.yml -> DEBUG=true.
• reload the API root page to see the django-debug-toolbar on the right
• navigate to /tasks/completed/{schedule_entry_name} and note that this page takes a long time to load
• use the SQL waterfall plot on django-debug-toolbar to analyze the query that took the most time
• improve it

[djanderson]