kgyrtkirk commented on a change in pull request #960: HIVE-23030 ds rollup union

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+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hive.ql.optimizer.calcite.functions;
+import org.apache.calcite.sql.SqlAggFunction;
+import org.apache.calcite.sql.SqlFunctionCategory;
+import org.apache.calcite.sql.SqlKind;
+import org.apache.calcite.sql.type.SqlOperandTypeChecker;
+import org.apache.calcite.sql.type.SqlOperandTypeInference;
+import org.apache.calcite.sql.type.SqlReturnTypeInference;
+ * Mergeable aggregate.
+ *
+ * A mergeable aggregate is:
+ * - accepts the same kind as inputs as the output (an X^n -> X function)
+ *
+ * Example: the SUM function is a great example; since SUM of SUM -s is the 
overall sum.
+ */
+public class HiveMergeablAggregate extends SqlAggFunction  {
 Review comment:
   That interface is quite wide; we might be able to utilize it - but to do 
that we should also implement functions like `singleton` (calculate the agg for 
a single value) which is trivial for a `COUNT` or a `SUM` (the interface is 
only implemented for things like these right now) ; but for a more complicated 
thing - like a sketch function it should involve executing the actual function 
to fold the single value.
   I think it would be better to have a set of smaller interfaces - instead of 
one bigger; that could make it easier to implement the most valuable ones for a 
given usecase; when the others are not neccessarily needed.
   This could be probably extended to support the `SqlSplittableAggFunction` 
interface - but as you can see there will be some challenges along the way ; so 
if we want to do that - I think it should at least have it's own ticket.

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