Nosql is very big. Compare redis + cassandra + elasticsearch and you'll see. You have to look at: data-structures the db uses, query patterns, data-velocity, da

Nosql is very big. Compare redis + cassandra + elasticsearch and you'll see. You have to look at: data-structures the db uses, query patterns, data-velocity, data-shape, etc etc. Basically how the db really works in a low level, to understand what is it good for. Current case they: have a lot of data, need a lot of compression (or low overhead as possible), need as much query-selectivity as possible (reading as few rows as possible for each query) that's why they have a unique case of selective indexes. Bonus: mongodb agg sucks. Seach online for comparison to pg or es agg.