[GitHub] [arrow] prouhard opened a new issue, #35343: DenseUnionVector is not transferred correctly

["prouhard (via GitHub)" <gi...@apache.org>]

prouhard opened a new issue, #35343:
URL: https://github.com/apache/arrow/issues/35343

   ### Describe the bug, including details regarding any error messages, version, and platform.
   
   Hello,
   
   I encountered a weird issue when converting a `Table` to a `VectorSchemaRoot`. All the `DenseUnionVector` were empty after the conversion.
   From my understanding, it looks like there is a bug in the `TransferImpl.transfer` method of the `DenseUnionVector`.
   After copying the `valueCount` value to the second vector [in this line](https://github.com/apache/arrow/blob/45918a90a6ca1cf3fd67c256a7d6a240249e555a/java/vector/src/main/codegen/templates/DenseUnionVector.java#L634), it should also set the `nextTypeId`.
   After the transfer, `nextTypeId` is equals to `0`, so trying to transfer again would create an empty vector as [the for loop is done against the `nextTypeId`](https://github.com/apache/arrow/blob/45918a90a6ca1cf3fd67c256a7d6a240249e555a/java/vector/src/main/codegen/templates/DenseUnionVector.java#L628)
   
   Proposed fix would be as simple as :
   
   ```java
       @Override
       public void transfer() {
   
         ...      
   
         for (int i = 0; i < nextTypeId; i++) {
           if (internalTransferPairs[i] != null) {
             internalTransferPairs[i].transfer();
             to.childVectors[i] = internalTransferPairs[i].getTo();
           }
         }
         to.valueCount = valueCount;
         to.nextTypeId = nextTypeId;
         clear();
       }
   ```
   
   I will raise a PR with the change.
   
   ### Component(s)
   
   Java


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