Github user gatorsmile commented on the issue:
https://github.com/apache/spark/pull/17277
Found a JIRA https://issues.apache.org/jira/browse/IMPALA-252 to explain
how IMPALA handles it.
**Static partition keys may not be NULL or the empty string**
So `INSERT INTO TABLE tbl PARTITION(part="") SELECT ...` will raise an
error.
**Dynamic partition keys may be empty or NULL**
So `INSERT INTO TABLE tbl PARTITION(part) SELECT ...`, `NULL` will work.
**Partitions with NULL or empty string keys are mapped to
`__HIVE_DEFAULT_PARTITION__`**
Whether the keys are `NULL` or "", both will be written to the same
`__HIVE_DEFAULT_PARTITION__` partition.
**Values read from the partitioned column in partition
__HIVE_DEFAULT_PARTITION__ are mapped back to NULL**
Here we deviate from Hive; Hive returns {{_HIVE_DEFAULT_PARTITION_ }} -
even if the partition column is of integer type. This finally crosses the line
of what we are willing to do to be compatible.
**ALTER TABLE [ADD|DROP] will reject partitions with NULL or empty
partition keys**
You cannot create or delete default partitions manually.
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