[SPARK-18622][SQL] Fix the datatype of the Sum aggregate function

## What changes were proposed in this pull request?
The result of a `sum` aggregate function is typically a Decimal, Double or a Long. Currently the output dataType is based on input's dataType.

The `FunctionArgumentConversion` rule will make sure that the input is promoted to the largest type, and that also ensures that the output uses a (hopefully) sufficiently large output dataType. The issue is that sum is in a resolved state when we cast the input type, this means that rules assuming that the dataType of the expression does not change anymore could have been applied in the mean time. This is what happens if we apply `WidenSetOperationTypes` before applying the casts, and this breaks analysis.

The most straight forward and future proof solution is to make `sum` always output the widest dataType in its class (Long for IntegralTypes, Decimal for DecimalTypes & Double for FloatType and DoubleType). This PR implements that solution.

We should move expression specific type casting rules into the given Expression at some point.

## How was this patch tested?
Added (regression) tests to SQLQueryTestSuite's `union.sql`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #16063 from hvanhovell/SPARK-18622.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Sum.scala

@@ -33,8 +33,7 @@ case class Sum(child: Expression) extends DeclarativeAggregate with ImplicitCast
   // Return data type.
   override def dataType: DataType = resultType
 
-  override def inputTypes: Seq[AbstractDataType] =
-    Seq(TypeCollection(LongType, DoubleType, DecimalType))
+  override def inputTypes: Seq[AbstractDataType] = Seq(NumericType)
 
   override def checkInputDataTypes(): TypeCheckResult =
     TypeUtils.checkForNumericExpr(child.dataType, "function sum")
@@ -42,7 +41,8 @@ case class Sum(child: Expression) extends DeclarativeAggregate with ImplicitCast
   private lazy val resultType = child.dataType match {
     case DecimalType.Fixed(precision, scale) =>
       DecimalType.bounded(precision + 10, scale)
-    case _ => child.dataType
+    case _: IntegralType => LongType
+    case _ => DoubleType
   }
 
   private lazy val sumDataType = resultType

sql/core/src/test/resources/sql-tests/inputs/union.sql

+CREATE OR REPLACE TEMPORARY VIEW t1 AS VALUES (1, 'a'), (2, 'b') tbl(c1, c2);
+CREATE OR REPLACE TEMPORARY VIEW t2 AS VALUES (1.0, 1), (2.0, 4) tbl(c1, c2);
+
+-- Simple Union
+SELECT *
+FROM   (SELECT * FROM t1
+        UNION ALL
+        SELECT * FROM t1);
+
+-- Type Coerced Union
+SELECT *
+FROM   (SELECT * FROM t1
+        UNION ALL
+        SELECT * FROM t2
+        UNION ALL
+        SELECT * FROM t2);
+
+-- Regression test for SPARK-18622
+SELECT a
+FROM (SELECT 0 a, 0 b
+      UNION ALL
+      SELECT SUM(1) a, CAST(0 AS BIGINT) b
+      UNION ALL SELECT 0 a, 0 b) T;
+
+-- Clean-up
+DROP VIEW IF EXISTS t1;
+DROP VIEW IF EXISTS t2;

sql/core/src/test/resources/sql-tests/results/union.sql.out

+-- Automatically generated by SQLQueryTestSuite
+-- Number of queries: 7
+
+
+-- !query 0
+CREATE OR REPLACE TEMPORARY VIEW t1 AS VALUES (1, 'a'), (2, 'b') tbl(c1, c2)
+-- !query 0 schema
+struct<>
+-- !query 0 output
+
+
+
+-- !query 1
+CREATE OR REPLACE TEMPORARY VIEW t2 AS VALUES (1.0, 1), (2.0, 4) tbl(c1, c2)
+-- !query 1 schema
+struct<>
+-- !query 1 output
+
+
+
+-- !query 2
+SELECT *
+FROM   (SELECT * FROM t1
+        UNION ALL
+        SELECT * FROM t1)
+-- !query 2 schema
+struct<c1:int,c2:string>
+-- !query 2 output
+1	a
+1	a
+2	b
+2	b
+
+
+-- !query 3
+SELECT *
+FROM   (SELECT * FROM t1
+        UNION ALL
+        SELECT * FROM t2
+        UNION ALL
+        SELECT * FROM t2)
+-- !query 3 schema
+struct<c1:decimal(11,1),c2:string>
+-- !query 3 output
+1	1
+1	1
+1	a
+2	4
+2	4
+2	b
+
+
+-- !query 4
+SELECT a
+FROM (SELECT 0 a, 0 b
+      UNION ALL
+      SELECT SUM(1) a, CAST(0 AS BIGINT) b
+      UNION ALL SELECT 0 a, 0 b) T
+-- !query 4 schema
+struct<a:bigint>
+-- !query 4 output
+0
+0
+1
+
+
+-- !query 5
+DROP VIEW IF EXISTS t1
+-- !query 5 schema
+struct<>
+-- !query 5 output
+
+
+
+-- !query 6
+DROP VIEW IF EXISTS t2
+-- !query 6 schema
+struct<>
+-- !query 6 output
+