[GitHub] [spark] amaliujia commented on a diff in pull request #36527: [SPARK-39169][SQL] Optimize FIRST when used as a single aggregate fun…

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amaliujia commented on code in PR #36527:
URL: https://github.com/apache/spark/pull/36527#discussion_r872689693


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/RewriteNonAggregateFirst.scala:
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@@ -0,0 +1,72 @@
+/*
+ * 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.catalyst.optimizer
+
+import org.apache.spark.sql.catalyst.expressions.{Alias, Literal}
+import org.apache.spark.sql.catalyst.expressions.aggregate.{AggregateExpression, First}
+import org.apache.spark.sql.catalyst.plans.logical._
+import org.apache.spark.sql.catalyst.rules.Rule
+import org.apache.spark.sql.catalyst.trees.TreePattern.FIRST
+import org.apache.spark.sql.types.IntegerType
+
+/**
+ * Rewrite aggregate plan with a single [[First]] function when grouping is absent. In such a case
+ * the query is equivalent to simple projection with limit 1.
+ *
+ * Input Pseudo-Query:
+ * {{{
+ *   SELECT FIRST(col) FROM table
+ * }}}
+ *
+ * Rewritten Query:
+ * {{{
+ *   SELECT col FROM table LIMIT 1

Review Comment:
   Why we want this optimization? Is the purpose to implement one less function? Are there other similar aggregation function that can apply the same idea?
   
   My concern is adding a rule to optimization process will increase the complexity of SQL compiler engine if the benefit is only to save implementation of a single aggregate function.  Conceptually having a aggregate function implemented  is much simpler. 



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