Optimize queries with indexing

[krishnans2006]

Django supports Model indexing, which allows for optimizing certain queries as explained here.

Some Tin queries are slow. The most notable example is submissions.Submission, which currently has >340,000 rows in production. Below are a few potentially-optimizable queries for the submissions app:

• submissions by student and assignment
• published submissions by student and assignment

Implementing indexing may (or may not) improve the performance of these queries, and this must be tested thoroughly to ensure real performance improvements.