Walton ,Dale and Jenevein

• They have presented a taxonomy of skew in the parallel databases Attribute Value skew : Inherent in the dataset . Partition skew : Occurs due to imbalance in the load between the nodes. AVS leads to partition skew , other factors include the following : (i) TPS : Tuple placement skew i.e. initial placement of tuples between nodes may vary. (ii)SS : Selectivity skew may vary from node to node for different predicates. (iii) RS : Redistribution skew occurs when tuples are redistributed in preparation for the actual join. (iv) JPS : Join Product skew occurs on individual nodes' varying selectivity leading to difference in number of output tuples.
• Walton et al used an analytical model to compare the scheduling hash join algorithm and the hybrid hash-join algo of Gamma .
• Result: Scheduling hash effectively handles RS while hybrid hash degrades and eventually becomes worse than scheduling hash as RS increases( significant skew only then ).