Github user unsleepy22 commented on the issue:
https://github.com/apache/flink/pull/2182
@wuchong although I agree this bug could be fixed by adding
`operands.nonEmpty`, there is a problem in generated code, specifically, an
undeclared variable will be generated like this:
```
long result$16 =
((org.apache.flink.api.java.table.TumblingWindowUDf)
udfDict.get("tumblingWin")).
eval()
;
if (isNull$17) {
out.setField(1, null);
}
else {
out.setField(1, result$16);
}
```
I guess it's generated in `GeneratedExpression`, for simplicity I reverted
my change.
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