HyukjinKwon commented on issue #26910: [SPARK-30154][ML] PySpark UDF to convert MLlib vectors to dense arrays URL: https://github.com/apache/spark/pull/26910#issuecomment-566842696 @cloud-fan, I know UDT became private and we plan to redesign it later. However, what about we allow the case when the UDT is cast into its own `sqlType`, or do you know why we don't allow this case? ```scala scala> val df = Seq(Tuple1(org.apache.spark.ml.linalg.Vectors.dense(1.0, 2.0, 3.0))).toDF("vec") df: org.apache.spark.sql.DataFrame = [vec: vector] scala> df.selectExpr("cast(vec as string)").show() +-------------+ | vec| +-------------+ |[1.0,2.0,3.0]| +-------------+ scala> df.selectExpr("cast(vec as struct<type:tinyint,size:int,indices:array<int>,values:array<double>>)").show() org.apache.spark.sql.AnalysisException: cannot resolve '`vec`' due to data type mismatch: cannot cast struct<type:tinyint,size:int,indices:array<int>,values:array<double>> to struct<type:tinyint,size:int,indices:array<int>,values:array<double>>; line 1 pos 0; 'Project [unresolvedalias(cast(vec#74 as struct<type:tinyint,size:int,indices:array<int>,values:array<double>>), None)] +- Project [_1#71 AS vec#74] +- LocalRelation [_1#71] at org.apache.spark.sql.catalyst.analysis.package$AnalysisErrorAt.failAnalysis(package.scala:42) at org.apache.spark.sql.catalyst.analysis.CheckAnalysis$$anonfun$$nestedInanonfun$checkAnalysis$1$2.applyOrElse(CheckAnalysis.scala:146) at org.apache.spark.sql.catalyst.analysis.CheckAnalysis$$anonfun$$nestedInanonfun$checkAnalysis$1$2.applyOrElse(CheckAnalysis.scala:137) at org.apache.spark.sql.catalyst.trees.TreeNode.$anonfun$transformUp$2(TreeNode.scala:310) at org.apache.spark.sql.catalyst.trees.CurrentOrigin$.withOrigin(TreeNode.scala:72) ``` Currently, UDT can be cast to string but cannot be its own `sqlType` (https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/ml/linalg/VectorUDT.scala#L88-L99). It's internally `InternalRow` so I think it seems fine to allow this case for now.
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