Github user lw-lin commented on the issue:
https://github.com/apache/spark/pull/14118
FYI, before
[SPARK-14143](https://issues.apache.org/jira/browse/SPARK-14143), null values
had been handled this way: :
```scala
if (datum == options.nullValue && nullable &&
(!castType.isInstanceOf[StringType]))
```
Then in [SPARK-14143](https://issues.apache.org/jira/browse/SPARK-14143),
it was first broken down into numeric data types in
https://github.com/apache/spark/pull/11947/commits/93ac6bb3eb63efb775b48af090a37a6cbe4f30c4
to handle byte-specific null value, short-specific null value, int-specific
null value, ... :
```scala
case _: ByteType => if (datum == params.byteNullValue && nullable) null
else datum.toByte
case _: ShortType => if (datum == params.shortNullValue && nullable) null
else datum.toShort
case _: IntegerType => if (datum == params.integerNullValue && nullable)
null else datum.toInt
...
```
then in
https://github.com/apache/spark/pull/11947/commits/698b4b41baa1ebd5d66ea6242bcb39bcd0887f8b
byte-specific null value, short-specific null value, int-specific null value,
... were reduced back to one single null value:
```scala
case _: ByteType => if (datum == params.nullValue && nullable) null else
datum.toByte
case _: ShortType => if (datum == params.nullValue && nullable) null else
datum.toShort
case _: IntegerType => if (datum == params.nullValue && nullable) null else
datum.toInt
```
Along with that change, we had introduced regression handling non-numeric
data types like `BooleanType` etc. Since we don't need to handle type-specific
null values, this patch switchs back to the way we handled null values in the
1.6 days (and thus fixes the regression):
```scala
if (datum == options.nullValue && nullable &&
(!castType.isInstanceOf[StringType]))
```
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