Joins are one of the fundamental relational database query operations. It is very hard to implement the join operation efficiently as there any many unknowns in the execution of the operation. In the early days much relation database research was done in understanding the complexity of performing joins, what exactly impacted their performance and which approach performed better under which conditions. In 1992 Priti Mishra and Margaret Eich conducted a survey on what was achieved until then in Join Processing and described in details the algorithms, the implementation complexity and the performance. Which make it a good back-to-basics paper to read this weekend.
Join Processing in Relational Databases, Priti Mishra and Margaret H. Eich, ACM Computing Surveys (CSUR) Surveys, Volume 24 Issue 1, March 1992, Pages 63 - 113