[SPARK-14563][ML] use a random table name instead of __THIS__ in SQLTransformer

## What changes were proposed in this pull request?

Use a random table name instead of `__THIS__` in SQLTransformer, and add a test for `transformSchema`. The problems of using `__THIS__` are:

* It doesn't work under HiveContext (in Spark 1.6)
* Race conditions

## How was this patch tested?

* Manual test with HiveContext.
* Added a unit test for `transformSchema` to improve coverage.

cc: yhuai

Author: Xiangrui Meng <meng@databricks.com>

Closes #12330 from mengxr/SPARK-14563.

1 files changed, 6 insertions, 4 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/ml/feature/SQLTransformer.scala b/mllib/src/main/scala/org/apache/spark/ml/feature/SQLTransformer.scala
index 95fe942c6b..2002d15745 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/feature/SQLTransformer.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/feature/SQLTransformer.scala
@@ -68,8 +68,7 @@ class SQLTransformer @Since("1.6.0") (override val uid: String) extends Transfor
     val tableName = Identifiable.randomUID(uid)
     dataset.registerTempTable(tableName)
     val realStatement = $(statement).replace(tableIdentifier, tableName)
-    val outputDF = dataset.sqlContext.sql(realStatement)
-    outputDF
+    dataset.sqlContext.sql(realStatement)
   }
 
   @Since("1.6.0")
@@ -78,8 +77,11 @@ class SQLTransformer @Since("1.6.0") (override val uid: String) extends Transfor
     val sqlContext = SQLContext.getOrCreate(sc)
     val dummyRDD = sc.parallelize(Seq(Row.empty))
     val dummyDF = sqlContext.createDataFrame(dummyRDD, schema)
-    dummyDF.registerTempTable(tableIdentifier)
-    val outputSchema = sqlContext.sql($(statement)).schema
+    val tableName = Identifiable.randomUID(uid)
+    val realStatement = $(statement).replace(tableIdentifier, tableName)
+    dummyDF.registerTempTable(tableName)
+    val outputSchema = sqlContext.sql(realStatement).schema
+    sqlContext.dropTempTable(tableName)
     outputSchema
   }