Technical Perspective: Accurate Summary-based Cardinality Estimation Through the Lens of Cardinality Estimation Graphs

Query engines are really good at choosing an efficient query plan. Users don't need to worry about how they write their query, since the optimizer makes all the right choices for executing the query, while taking into account all aspects of data, such as its size, the characteristics of the storage device, the distribution pattern, the availability of indexes, and so on. The query optimizer always makes the best choice, no matter how complex the query is, or how contrived it was written. Or, this is what we expect today from a modern query optimizer. Unfortunately, reality is not as nice.