Github user jinfengni commented on a diff in the pull request:
https://github.com/apache/drill/pull/906#discussion_r135974107
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/project/ProjectRecordBatch.java
---
@@ -768,4 +765,73 @@ else if (exprHasPrefix && refHasPrefix) {
}
}
}
+
+ /**
+ * handle FAST NONE specially when Project for query output. This
happens when input returns a
+ * FAST NONE directly ( input does not return any batch with
schema/data).
+ *
+ * Project operator has to return a batch with schema derived using the
following 3 rules:
+ * Case 1: * ==> expand into an empty list of columns.
+ * Case 2: regular column reference ==> treat as nullable-int column
+ * Case 3: expressions => Call ExpressionTreeMaterialization over an
empty vector contain.
--- End diff --
I managed to find a way to share the existing code path. With the revised
patch, both of the first scenario and second one would call same method to
setup a new schema based on incoming batch. The only difference for the second
scenario is we have to simulate a record batch with empty vector container, as
the code for set up new schema requires it. That is done by leveraging an
existing class (with some extension).
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