Github user dongjoon-hyun commented on the pull request:
https://github.com/apache/spark/pull/12340#issuecomment-210280000
It's Hive built-in function. The following is function description.
```
hive> desc function assert_true;
OK
assert_true(condition) - Throw an exception if 'condition' is not true.
Time taken: 0.022 seconds, Fetched: 1 row(s)
```
Let me find some documentation more.
By the way, when I make a UDF return NULL, it shows the same behavior.
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