[SPARK-16714][SPARK-16735][SPARK-16646] array, map, greatest, least's type coercion should handle decimal type

## What changes were proposed in this pull request?

Here is a table about the behaviours of `array`/`map` and `greatest`/`least` in Hive, MySQL and Postgres:

|    |Hive|MySQL|Postgres|
|---|---|---|---|---|
|`array`/`map`|can find a wider type with decimal type arguments, and will truncate the wider decimal type if necessary|can find a wider type with decimal type arguments, no truncation problem|can find a wider type with decimal type arguments, no truncation problem|
|`greatest`/`least`|can find a wider type with decimal type arguments, and truncate if necessary, but can't do string promotion|can find a wider type with decimal type arguments, no truncation problem, but can't do string promotion|can find a wider type with decimal type arguments, no truncation problem, but can't do string promotion|

I think these behaviours makes sense and Spark SQL should follow them.

This PR fixes `array` and `map` by using `findWiderCommonType` to get the wider type.
This PR fixes `greatest` and `least` by add a `findWiderTypeWithoutStringPromotion`, which provides similar semantic of `findWiderCommonType`, but without string promotion.

## How was this patch tested?

new tests in `TypeCoersionSuite`

Author: Wenchen Fan <wenchen@databricks.com>
Author: Yin Huai <yhuai@databricks.com>

Closes #14439 from cloud-fan/bug.

1 files changed, 30 insertions, 17 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
index 8503b8dcf8..021952e716 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
@@ -109,18 +109,6 @@ object TypeCoercion {
   }
 
   /**
-   * Similar to [[findTightestCommonType]], if can not find the TightestCommonType, try to use
-   * [[findTightestCommonTypeToString]] to find the TightestCommonType.
-   */
-  private def findTightestCommonTypeAndPromoteToString(types: Seq[DataType]): Option[DataType] = {
-    types.foldLeft[Option[DataType]](Some(NullType))((r, c) => r match {
-      case None => None
-      case Some(d) =>
-        findTightestCommonTypeToString(d, c)
-    })
-  }
-
-  /**
    * Find the tightest common type of a set of types by continuously applying
    * `findTightestCommonTypeOfTwo` on these types.
    */
@@ -157,6 +145,28 @@ object TypeCoercion {
     })
   }
 
+  /**
+   * Similar to [[findWiderCommonType]], but can't promote to string. This is also similar to
+   * [[findTightestCommonType]], but can handle decimal types. If the wider decimal type exceeds
+   * system limitation, this rule will truncate the decimal type before return it.
+   */
+  private def findWiderTypeWithoutStringPromotion(types: Seq[DataType]): Option[DataType] = {
+    types.foldLeft[Option[DataType]](Some(NullType))((r, c) => r match {
+      case Some(d) => findTightestCommonTypeOfTwo(d, c).orElse((d, c) match {
+        case (t1: DecimalType, t2: DecimalType) =>
+          Some(DecimalPrecision.widerDecimalType(t1, t2))
+        case (t: IntegralType, d: DecimalType) =>
+          Some(DecimalPrecision.widerDecimalType(DecimalType.forType(t), d))
+        case (d: DecimalType, t: IntegralType) =>
+          Some(DecimalPrecision.widerDecimalType(DecimalType.forType(t), d))
+        case (_: FractionalType, _: DecimalType) | (_: DecimalType, _: FractionalType) =>
+          Some(DoubleType)
+        case _ => None
+      })
+      case None => None
+    })
+  }
+
   private def haveSameType(exprs: Seq[Expression]): Boolean =
     exprs.map(_.dataType).distinct.length == 1
 
@@ -440,7 +450,7 @@ object TypeCoercion {
 
       case a @ CreateArray(children) if !haveSameType(children) =>
         val types = children.map(_.dataType)
-        findTightestCommonTypeAndPromoteToString(types) match {
+        findWiderCommonType(types) match {
           case Some(finalDataType) => CreateArray(children.map(Cast(_, finalDataType)))
           case None => a
         }
@@ -451,7 +461,7 @@ object TypeCoercion {
           m.keys
         } else {
           val types = m.keys.map(_.dataType)
-          findTightestCommonTypeAndPromoteToString(types) match {
+          findWiderCommonType(types) match {
             case Some(finalDataType) => m.keys.map(Cast(_, finalDataType))
             case None => m.keys
           }
@@ -461,7 +471,7 @@ object TypeCoercion {
           m.values
         } else {
           val types = m.values.map(_.dataType)
-          findTightestCommonTypeAndPromoteToString(types) match {
+          findWiderCommonType(types) match {
             case Some(finalDataType) => m.values.map(Cast(_, finalDataType))
             case None => m.values
           }
@@ -494,16 +504,19 @@ object TypeCoercion {
           case None => c
         }
 
+      // When finding wider type for `Greatest` and `Least`, we should handle decimal types even if
+      // we need to truncate, but we should not promote one side to string if the other side is
+      // string.g
       case g @ Greatest(children) if !haveSameType(children) =>
         val types = children.map(_.dataType)
-        findTightestCommonType(types) match {
+        findWiderTypeWithoutStringPromotion(types) match {
           case Some(finalDataType) => Greatest(children.map(Cast(_, finalDataType)))
           case None => g
         }
 
       case l @ Least(children) if !haveSameType(children) =>
         val types = children.map(_.dataType)
-        findTightestCommonType(types) match {
+        findWiderTypeWithoutStringPromotion(types) match {
           case Some(finalDataType) => Least(children.map(Cast(_, finalDataType)))
           case None => l
         }