Josh Rosen commented on SPARK-17618:

Yep, the problem is that {{Coalesce}} advertises that it accepts Unsafe rows 
but misdeclares its row output format as being regular rows. Comparing an 
UnsafeRow to any other row type for equality always returns false (its 
{{equals()}} implementation is compatible with Java universal equality, so it 
doesn't throw when performing a comparison against a different type). As a 
result, the Except compares safe and unsafe rows, causing the comparisons to be 
incorrect and leading to the wrong answer that you saw here.

I'm marking this as a blocker for 1.6.3 and am working on a fix which will fix 
this issue.

> Dataframe except returns incorrect results when combined with coalesce
> ----------------------------------------------------------------------
>                 Key: SPARK-17618
>                 URL: https://issues.apache.org/jira/browse/SPARK-17618
>             Project: Spark
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 1.6.1, 1.6.2
>            Reporter: Graeme Edwards
>            Priority: Blocker
>              Labels: correctness
> We were getting incorrect results from the DataFrame except method - all rows 
> were being returned instead of the ones that intersected. Calling subtract on 
> the underlying RDD returned the correct result.
> We tracked it down to the use of coalesce - the following is the simplest 
> example case we created that reproduces the issue:
> {code}
> val schema = new StructType().add("test", types.IntegerType )
> val t1 = sql.createDataFrame(sql.sparkContext.parallelize(1 to 100).map(i=> 
> Row(i)), schema)
> val t2 = sql.createDataFrame(sql.sparkContext.parallelize(5 to 10).map(i=> 
> Row(i)), schema)
> val t3 = t1.join(t2, t1.col("test").equalTo(t2.col("test")), "leftsemi")
> println("Count using normal except = " + t1.except(t3).count())
> println("Count using coalesce = " + 
> t1.coalesce(8).except(t3.coalesce(8)).count())
> {code}
> We should get the same result from both uses of except, but the one using 
> coalesce returns 100 instead of 94.

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