[SPARK-5203][SQL] fix union with different decimal type

   When union non-decimal types with decimals, we use the following rules:
      - FIRST `intTypeToFixed`, then fixed union decimals with precision/scale p1/s2 and p2/s2  will be promoted to
      DecimalType(max(p1, p2), max(s1, s2))
      - FLOAT and DOUBLE cause fixed-length decimals to turn into DOUBLE (this is the same as Hive,
      but note that unlimited decimals are considered bigger than doubles in WidenTypes)

Author: guowei2 <guowei2@asiainfo.com>

Closes #4004 from guowei2/SPARK-5203 and squashes the following commits:

ff50f5f [guowei2] fix code style
11df1bf [guowei2] fix decimal union with double, double->Decimal(15,15)
0f345f9 [guowei2] fix structType merge with decimal
101ed4d [guowei2] fix build error after rebase
0b196e4 [guowei2] code style
fe2c2ca [guowei2] handle union decimal precision in 'DecimalPrecision'
421d840 [guowei2] fix union types for decimal precision
ef2c661 [guowei2] fix union with different decimal type

1 files changed, 11 insertions, 0 deletions

diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala
index 2065f0d60d..817b9dcb8f 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/SQLQuerySuite.scala
@@ -468,4 +468,15 @@ class SQLQuerySuite extends QueryTest {
       sql(s"DROP TABLE $tableName")
     }
   }
+  
+  test("SPARK-5203 union with different decimal precision") {
+    Seq.empty[(Decimal, Decimal)]
+      .toDF("d1", "d2")
+      .select($"d1".cast(DecimalType(10, 15)).as("d"))
+      .registerTempTable("dn")
+
+    sql("select d from dn union all select d * 2 from dn")
+      .queryExecution.analyzed
+  }
+
 }