In RDBMS, goals of Normalization:Free the database of modification anomaliesMinimize re-design when extendingAvoid bias toward any particular access patternFirst step involves avoiding redundancy etc...As data is normalised and divided in multiple tables, join operations are required to satisfy queries placed by application. join operation is costly.With relational model, there is an issue of  object mismatch i.e., gap between real world(application access) and relational world.On the contrast, Document databases(like MongoDB) do not have object mismatch issue. The single most important factor in designing the database schema with document database, is about, matching the data access patterns of your application. So, database design is pretty easy.They also support transactions, nowDe-normalisation issue still gets carried in Document database, unless you link data among collections and map/reduce on application accessQuestion:Why document oriented database is yet to replace relational model database?
Normalisation Vs Join , trade-off
database;mysql;mongodb;postgres;normalization