[IMPALA-5580] Java UDF: return null STRING incorrectly converted to empty string

• Type: Bug
• Status: Resolved
• Priority: Blocker
• Resolution: Fixed
• Affects Version/s: Impala 2.8.0
• Fix Version/s: Impala 2.10.0
• Component/s: Backend
• Labels: correctness ramp-up
• Target Version: Impala 2.10.0
• Epic Color: ghx-label-7
• Assignee: Daniel Hecht
• Reporter: Lorenzo Bagni
• Created: 26/Jun/17 17:18
• Updated: 07/Jul/17 16:06
• Resolved: 07/Jul/17 16:04

Description

Hi all,
I'm facing on wrong behaviour using Java UDF, when i declare a 'return null;' inside the code and I call the function the value returned is a blank (like '' or "") value.
This happen when the input type is a STRING, just to be sure I've used the TestUdf:
http://impala.io/doc/html/TestUdf_8java_source.html
here's my tests:

[host:21000] > select testudf(cast(null as int));
Query: select testudf(cast(null as int))
--------------------------------+
| dwh.testudf(cast(null as int)) |
+--------------------------------+
| NULL                           |
+--------------------------------+
Fetched 1 row(s) in 0.12s
[host:21000] > select testudf(cast(null as string));
Query: select testudf(cast(null as string))
+-----------------------------------+
| dwh.testudf(cast(null as string)) |
+-----------------------------------+
|                                   |
+-----------------------------------+
Fetched 1 row(s) in 0.15s
[host:21000] > select testudf(cast(null as string)) is null;
Query: select testudf(cast(null as string)) is null
+-------------------------------------------+
| dwh.testudf(cast(null as string)) is null |
+-------------------------------------------+
| false                                     |
+-------------------------------------------+
Fetched 1 row(s) in 0.12s
[host:21000] > select testudf(cast(null as int)) is null;
Query: select testudf(cast(null as int)) is null
+----------------------------------------+
| dwh.testudf(cast(null as int)) is null |
+----------------------------------------+
| true                                   |
+----------------------------------------+
Fetched 1 row(s) in 0.15s

On impala v2.6.0 the same UDF works correcty.

Issue Links

• is broken by: IMPALA-5252 Java UDF returning string can lead to crash under memory pressure.