Github user paul-rogers commented on a diff in the pull request:
https://github.com/apache/drill/pull/713#discussion_r94814315
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/flatten/FlattenRecordBatch.java
---
@@ -305,12 +306,23 @@ protected boolean setupNewSchema() throws
SchemaChangeException {
final NamedExpression flattenExpr = new
NamedExpression(popConfig.getColumn(), new
FieldReference(popConfig.getColumn()));
final ValueVectorReadExpression vectorRead =
(ValueVectorReadExpression)ExpressionTreeMaterializer.materialize(flattenExpr.getExpr(),
incoming, collector, context.getFunctionRegistry(), true);
- final TransferPair tp =
getFlattenFieldTransferPair(flattenExpr.getRef());
-
- if (tp != null) {
- transfers.add(tp);
- container.add(tp.getTo());
- transferFieldIds.add(vectorRead.getFieldId().getFieldIds()[0]);
+ final FieldReference fieldReference = flattenExpr.getRef();
+ final TransferPair transferPair =
getFlattenFieldTransferPair(fieldReference);
+
+ if (transferPair != null) {
+ final ValueVector flattenVector = transferPair.getTo();
+
+ // checks that list has only default ValueVector and replaces
resulting ValueVector to INT typed ValueVector
+ if (exprs.size() == 0 &&
flattenVector.getField().getType().equals(Types.LATE_BIND_TYPE)) {
+ final MaterializedField outputField =
MaterializedField.create(fieldReference.getAsNamePart().getName(),
Types.OPTIONAL_INT);
+ final ValueVector vector = TypeHelper.getNewVector(outputField,
oContext.getAllocator());
--- End diff --
The fix appears to be to transform an empty list into an empty list of
integers. That is, Drill does not have the concept of "empty list", only "empty
list of type X" and we are guessing the type to be integer.
We've had issues elsewhere in the product where such guesses turn out to be
wrong. Perhaps the next row/batch has a non-empty list, but of strings. Or
worse, of objects (maps.) Downstream operators cannot handle this.
The result is that a query fails for no better reason than we caused it to
fail by guessing the wrong type.
Clearly, fixing the broader problem is beyond the scope of this fix. I am
pointing out, however, that a consequence of the assumptirnmade here is that
some queries, somewhere later, will fail due to an artificial schema change.
The correct solution is to introduce an "Unknown" type and mark this a
vector of type "Unknown". All we know is that it is a list; the member types
are unknown. Then, in downstream operators, when we encounter a schema change,
we know that an empty list of "Unknown" type is compatible with a list of any
other type (say maps.)
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