[GitHub] [spark] holdenk commented on a change in pull request #24559: [SPARK-27658][SQL] Add FunctionCatalog API

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holdenk commented on a change in pull request #24559:
URL: https://github.com/apache/spark/pull/24559#discussion_r500624589



##########
File path: sql/catalyst/src/main/java/org/apache/spark/sql/catalog/v2/AggregateFunction.java
##########
@@ -0,0 +1,68 @@
+/*
+ * 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
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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.spark.sql.catalog.v2;
+
+import org.apache.spark.sql.catalyst.InternalRow;
+import org.apache.spark.sql.types.DataType;
+
+/**
+ * Interface for a function that produces a result value by aggregating over multiple input rows.
+ * <p>
+ * The JVM type of result values produced by this function must be the type used by Spark's
+ * InternalRow API for the {@link DataType SQL data type} returned by {@link #resultType()}.
+ *
+ * @param <S> the JVM type for the aggregation's intermediate state
+ * @param <R> the JVM type of result values
+ */
+public interface AggregateFunction<S, R> extends BoundFunction {
+
+  /**
+   * Initialize state for an aggregation.
+   * <p>
+   * This method is called one or more times for every group of values to initialize intermediate
+   * aggregation state. More than one intermediate aggregation state variable may be used when the
+   * aggregation is run in parallel tasks.
+   * <p>
+   * The object returned may passed to {@link #update(Object, InternalRow)},
+   * and {@link #produceResult(Object)}. Implementations that return null must support null state
+   * passed into all other methods.
+   *
+   * @return a state instance or null
+   */
+  S newAggregationState();
+
+  /**
+   * Update the aggregation state with a new row.
+   * <p>
+   * This is called for each row in a group to update an intermediate aggregation state.
+   *
+   * @param state intermediate aggregation state
+   * @param input an input row
+   * @return updated aggregation state
+   */
+  S update(S state, InternalRow input);

Review comment:
       Would it make sense to have a default implementation for taking an iterator of internalrows? Just thinking out loud.

##########
File path: sql/catalyst/src/main/java/org/apache/spark/sql/catalog/v2/AggregateFunction.java
##########
@@ -0,0 +1,68 @@
+/*
+ * 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
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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.spark.sql.catalog.v2;
+
+import org.apache.spark.sql.catalyst.InternalRow;
+import org.apache.spark.sql.types.DataType;
+
+/**
+ * Interface for a function that produces a result value by aggregating over multiple input rows.

Review comment:
       Just thinking back to our initial groupByKey impl, can we add a warning here that if folks do not implement the `AssociativeAggregateFunction` they are going to force all values for a key onto a single node.




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