Optimize DB indexes

[ffont]

Currently we use tons of GB on database indexes. We should check if all of these are necessary and whether we can get rid of some of them. Maybe postgreSQL has a way to report in which indexes are used and which are not...

[alastair]

pg_stat_statements lets us see which types of queries are slow and need indexes: https://www.postgresql.org/docs/current/pgstatstatements.html

Unused indexes can be found with https://pgdash.io/blog/finding-unused-indexes-in-postgresql.html