Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/11190#discussion_r53514624
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/subquery.scala
---
@@ -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.catalyst.expressions
+
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
+import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback
+import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
+import org.apache.spark.sql.types.DataType
+
+/**
+ * A interface for subquery that is used in expressions.
+ */
+trait SubqueryExpression extends LeafExpression {
+ def query: LogicalPlan
+ def withNewPlan(plan: LogicalPlan): SubqueryExpression
+}
+
+/**
+ * A subquery that will return only one row and one column.
+ */
+case class ScalarSubquery(query: LogicalPlan) extends SubqueryExpression
with CodegenFallback {
+
+ override lazy val resolved: Boolean = query.resolved
+
+ override def dataType: DataType = query.schema.fields.head.dataType
+
+ override def checkInputDataTypes(): TypeCheckResult = {
+ if (query.schema.length != 1) {
+ TypeCheckResult.TypeCheckFailure("Scalar subquery can only have 1
column, but got " +
+ query.schema.length.toString)
+ } else {
+ TypeCheckResult.TypeCheckSuccess
+ }
+ }
+
+ // It can not be evaluated by optimizer.
+ override def foldable: Boolean = false
+ override def nullable: Boolean = true
+
+ override def withNewPlan(plan: LogicalPlan): ScalarSubquery =
ScalarSubquery(plan)
+
+ // TODO: support sql()
+
+ // the first column in first row from `query`.
+ private var result: Any = null
+
+ def updateResult(v: Any): Unit = {
--- End diff --
scaladoc
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