Re: [PR] [SPARK-52281][SQL] Change `ALTER TABLE ALTER COLUMN TYPE STRING` not to apply default collation if original data type was instance of `StringType` [spark]

[via GitHub]

cloud-fan commented on code in PR #51001:
URL: https://github.com/apache/spark/pull/51001#discussion_r2110780641


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/ApplyDefaultCollationToStringType.scala:
##########
@@ -151,22 +150,82 @@ object ApplyDefaultCollationToStringType extends 
Rule[LogicalPlan] {
       case p if isCreateOrAlterPlan(p) || 
AnalysisContext.get.collation.isDefined =>
         transformPlan(p, newType)
 
-      case addCols: AddColumns =>
+      case addCols@AddColumns(_: ResolvedTable, _) =>
         addCols.copy(columnsToAdd = replaceColumnTypes(addCols.columnsToAdd, 
newType))
 
-      case replaceCols: ReplaceColumns =>
+      case replaceCols@ReplaceColumns(_: ResolvedTable, _) =>
         replaceCols.copy(columnsToAdd = 
replaceColumnTypes(replaceCols.columnsToAdd, newType))
 
-      case a @ AlterColumns(_, specs: Seq[AlterColumnSpec]) =>
+      case a @ AlterColumns(ResolvedTable(_, _, table: Table, _), specs: 
Seq[AlterColumnSpec]) =>
         val newSpecs = specs.map {
-          case spec if spec.newDataType.isDefined && 
hasDefaultStringType(spec.newDataType.get) =>
+          case spec if shouldApplyDefaultCollationToAlterColumn(spec, table) =>
             spec.copy(newDataType = 
Some(replaceDefaultStringType(spec.newDataType.get, newType)))
           case col => col
         }
         a.copy(specs = newSpecs)
     }
   }
 
+  /**
+   * The column type should not be changed if the original column type is 
[[StringType]] and the new
+   * type is the default [[StringType]] (i.e., [[StringType]] without an 
explicit collation).
+   *
+   * Query Example:
+   * {{{
+   *   CREATE TABLE t (c1 STRING COLLATE UNICODE)
+   *   ALTER TABLE t ALTER COLUMN c1 TYPE STRING -- c1 will remain STRING 
COLLATE UNICODE

Review Comment:
   cc @srielau 



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