strongduanmu commented on issue #21417:
URL:
https://github.com/apache/shardingsphere/issues/21417#issuecomment-1272714410
l have do some research in calcite, when we execute `SELECT * FROM t_order
UNION ALL SELECT * FROM t_order;`, calcite will generate following java code.
```
public org.apache.calcite.linq4j.Enumerable bind(final
org.apache.calcite.DataContext root) {
final org.apache.calcite.linq4j.Enumerable child0 =
org.apache.calcite.schema.Schemas.enumerable((org.apache.calcite.schema.ProjectableFilterableTable)
root.getRootSchema().getSubSchema("public").getTable("t_order"), root);
return child0.concat(child0);
}
public Class getElementType() {
return java.lang.Object[].class;
}
```
Since both select statements are same, so calcite generate
`child0.concat(child0)` to handle this logic, and child0 corresponds to the
result set returned after pushdown. Fetch two identical objects will cause the
second result set to fail to be accessed normally, and the query result is
empty, so the UNION result return wrong.
<img width="1156" alt="image"
src="https://user-images.githubusercontent.com/10829171/194790741-0e84f102-ad27-4a73-9dfa-8b9ad1409c8d.png";>
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