cloud-fan commented on a change in pull request #26716: [SPARK-30083][SQL] 
visitArithmeticUnary should wrap PLUS case with UnaryPositive for type checking
URL: https://github.com/apache/spark/pull/26716#discussion_r353111472
 
 

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 File path: docs/sql-migration-guide.md
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 @@ -254,6 +254,8 @@ license: |
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+    - Since Spark 3.0, the unary arithmetic operator plus(`+`) only accepts 
string, numeric and interval type values as inputs. Besides, `+` with a 
integral string representation will be coerced to double value, e.g. `+'1'` 
results `1.0`. In Spark version 2.4 and earlier,  there is no type checking for 
it, thus, all type values with a `+` prefix are valid, e.g. `+ array(1, 2)` is 
valid and results `[1, 2]`. Besides, there is no implicit type coercion for `+` 
with string, e.g. in Spark 2.4, the result of `+'1'` is string `1`.
 
 Review comment:
   BTW it's worth mention that, in 2.4 the unary plus operator is ignored. 
There is no type coercion at all, not only for string type.

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