Re:Bye Bye Mongo, Hello Postgres
by manigandham on Wednesday December 19, 2018 @05:53PM
(#18720688)
It's not, it's a strategy to improve the performance of a particular query or access pattern, and is usually the last resort after things like proper indexing, aggregations and materialized views.
JOINS are fast and it all comes down to how much data you're moving. If it's a large table joining to a small set of values, then the joined data is quickly retrieved and applied to the bigger table, with great performance.
If the join is between two large tables where every combination is unique then that's the unique case where the joined table is just adding another hop to get the full set of data for each row and is a perfect candidate for denormalization, although in that case it probably should've been a single table to begin with. Of course there's a spectrum between these two scenarios but it takes a lot before denormalization makes sense on any modern RDBMS.
JOINS are fast and it all comes down to how much data you're moving. If it's a large table joining to a small set of values, then the joined data is quickly retrieved and applied to the bigger table, with great performance.
If the join is between two large tables where every combination is unique then that's the unique case where the joined table is just adding another hop to get the full set of data for each row and is a perfect candidate for denormalization, although in that case it probably should've been a single table to begin with. Of course there's a spectrum between these two scenarios but it takes a lot before denormalization makes sense on any modern RDBMS.