默认为10MB，如果大于该值不会执行mapjoin，hive语句中直接设置的mapjoin也不再起作用。
参考hive wiki把hive.auto.convert.join.noconditionaltask.size
修改大一些就ok。
官方解释为：
hive.auto.convert.join.noconditionaltask
* Default Value: true
* Added In: 0.11.0 with HIVE-3784 (default changed to true with HIVE-4146)
Whether Hive enables the optimization about converting common join into mapjoin based on the input file size. If this parameter is on, and the sum of size for n-1 of the tables/partitions for an n-way join is smaller than the size specified by hive.auto.convert.join.noconditionaltask.size, the join is directly converted to a mapjoin (there is no conditional task).
hive.auto.convert.join.noconditionaltask.size
* Default Value: 10000000
* Added In: 0.11.0 with HIVE-3784
If hive.auto.convert.join.noconditionaltask is off, this parameter does not take effect. However, if it is on, and the sum of size for n-1 of the tables/partitions for an n-way join is smaller than this size, the join is directly converted to a mapjoin (there is no conditional task). The default is 10MB.
https://cwiki.apache.org/confluence/display/Hive/Configuration+Properties