In the hive, the (enabled Map join) a large table left join a small table, loaded from right to left, so the small table is loaded into memory, key-value pairs stored as map, table-driven small table by the large, to join, that is, large table join field as a key to get the value carried join.
In MySQL, left join loaded from left to right, that join the left of the table will be loaded into memory, perform join with the right table.
When mapjoin make small table into memory, large tables into memory, then the reason may be because of too much, resulting in mapjoin can not be achieved, and switch to reduce join.
MySQL hive loaded with a different order.