Research on how to handle skewed join for hive on spark. Here is original hive's design doc for skewed join, https://cwiki.apache.org/confluence/display/Hive/Skewed+Join+Optimization.
Research optimization of auto convert join to map join [Spark branch]
Make sure map join tasks created by runtime skew join can fit into memory [Spark Branch]
Enable compile time skew join optimization for spark [Spark Branch]
Enable SkewJoinResolver for spark [Spark Branch]
Make SparkMapJoinResolver handle runtime skew join [Spark Branch]
Support runtime skew join for more queries [Spark Branch]