Apache Hive Essentials

Special joins

HQL also supports some special joins that we usually do not see in relational databases, such as MapJoin and Semi-join. MapJoin means doing the join operation only with map, without the reduce job. The MapJoin statement reads all the data from the small table to memory and broadcasts to all maps. During the map phase, the join operation is performed by comparing each row of data in the big table with small tables against the join conditions. Because there is no reduce needed, such kinds of join usually have better performance. In the newer version of Hive, Hive automatically converts join to MapJoin at runtime if possible. However, you can also manually specify the broadcast table by providing a join hint, /*+ MAPJOIN(table_name) ...