Github user yhuai commented on a diff in the pull request:
https://github.com/apache/spark/pull/4004#discussion_r22815717
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/HiveTypeCoercion.scala
---
@@ -154,6 +165,15 @@ trait HiveTypeCoercion {
*
* Additionally, all types when UNION-ed with strings will be promoted
to strings.
* Other string conversions are handled by PromoteStrings.
+ * when mixing non-decimal types with decimals, we use the following
rules:
+ * - BYTE gets turned into DECIMAL(3, 0)
+ * - SHORT gets turned into DECIMAL(5, 0)
+ * - INT gets turned into DECIMAL(10, 0)
+ * - LONG gets turned into DECIMAL(20, 0)
+ * - Fixed union decimals with precision/scale p1/s2 and p2/s2 will
be promoted to
+ * DecimalType(max(p1, p2), max(s1, s2))
--- End diff --
Can you add the reference of this rule (the rule for union)? I checked
http://msdn.microsoft.com/en-us/library/ms190476.aspx (I believe that Hive is
based on this one). Seems the type will be
```
DecimalType(max(s1, s2) + max(p1-s1, p2-s2), max(s1, s2))
```
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