Github user yjshen commented on the pull request:
https://github.com/apache/flink/pull/1880#issuecomment-209954946
I happened to have context here for `primitiveDefaultValue`, I think this
was originated from Apache Spark's counterpart for create default values for an
expression. Let me summarize how it is used to calculate scalar expression's
result in Spark:
```
boolean ${result.isNull} = false;
$javaType ${result.value} = ${ctx.defaultValue(type)};
if (result is not null for some input) {
${result.value} = calculated result;
} else {
${result.isNull} = true;
}
```
Therefore, after the evaluation of this expression, if the result is Null,
the `isNull` field is set to `True` and no more touches on `value` field, if
not null, the `result.value` point to the final result. The default value is
never used.
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