[SPARK-49843][SQL] Fix change comment on char/varchar columns

### What changes were proposed in this pull request?

Fix the issue in `AlterTableChangeColumnCommand` where changing the comment of a char/varchar column also tries to change the column type to string.

### Why are the changes needed?

Because the newColumn will always be a `StringType` even when the metadata says that it was originally char/varchar.

### Does this PR introduce _any_ user-facing change?

Yes, the query will no longer fail when using this code path.

### How was this patch tested?

New query in golden files.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #48315 from stefankandic/fixAlterVarcharCol.

Authored-by: Stefan Kandic <[email protected]>
Signed-off-by: Max Gekk <[email protected]>

sql/core/src/main/scala/org/apache/spark/sql/catalyst/analysis/ResolveSessionCatalog.scala

@@ -26,7 +26,7 @@ import org.apache.spark.sql.catalyst.catalog.{CatalogStorageFormat, CatalogTable
 import org.apache.spark.sql.catalyst.expressions.{Alias, Attribute}
 import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.catalyst.rules.Rule
-import org.apache.spark.sql.catalyst.util.{quoteIfNeeded, toPrettySQL, ResolveDefaultColumns => DefaultCols}
+import org.apache.spark.sql.catalyst.util.{quoteIfNeeded, toPrettySQL, CharVarcharUtils, ResolveDefaultColumns => DefaultCols}
 import org.apache.spark.sql.catalyst.util.ResolveDefaultColumns._
 import org.apache.spark.sql.connector.catalog.{CatalogExtension, CatalogManager, CatalogPlugin, CatalogV2Util, LookupCatalog, SupportsNamespaces, V1Table}
 import org.apache.spark.sql.connector.expressions.Transform
@@ -36,7 +36,7 @@ import org.apache.spark.sql.execution.datasources.{CreateTable => CreateTableV1}
 import org.apache.spark.sql.execution.datasources.v2.DataSourceV2Utils
 import org.apache.spark.sql.internal.{HiveSerDe, SQLConf}
 import org.apache.spark.sql.internal.connector.V1Function
-import org.apache.spark.sql.types.{MetadataBuilder, StructField, StructType}
+import org.apache.spark.sql.types.{MetadataBuilder, StringType, StructField, StructType}
 import org.apache.spark.util.ArrayImplicits._
 
 /**
@@ -87,7 +87,11 @@ class ResolveSessionCatalog(val catalogManager: CatalogManager)
       val colName = a.column.name(0)
       val dataType = a.dataType.getOrElse {
         table.schema.findNestedField(Seq(colName), resolver = conf.resolver)
-          .map(_._2.dataType)
+          .map {
+            case (_, StructField(_, st: StringType, _, metadata)) =>
+              CharVarcharUtils.getRawType(metadata).getOrElse(st)
+            case (_, field) => field.dataType
+          }
           .getOrElse {
             throw QueryCompilationErrors.unresolvedColumnError(
               toSQLId(a.column.name), table.schema.fieldNames)

sql/core/src/test/resources/sql-tests/analyzer-results/charvarchar.sql.out

@@ -263,6 +263,18 @@ desc formatted char_part
 DescribeTableCommand `spark_catalog`.`default`.`char_part`, true, [col_name#x, data_type#x, comment#x]
 
 
+-- !query
+alter table char_part change column c1 comment 'char comment'
+-- !query analysis
+AlterTableChangeColumnCommand `spark_catalog`.`default`.`char_part`, c1, StructField(c1,CharType(5),true)
+
+
+-- !query
+alter table char_part change column v1 comment 'varchar comment'
+-- !query analysis
+AlterTableChangeColumnCommand `spark_catalog`.`default`.`char_part`, v1, StructField(v1,VarcharType(6),true)
+
+
 -- !query
 alter table char_part add partition (v2='ke', c2='nt') location 'loc1'
 -- !query analysis

sql/core/src/test/resources/sql-tests/inputs/charvarchar.sql

@@ -49,6 +49,8 @@ desc formatted char_tbl1;
 create table char_part(c1 char(5), c2 char(2), v1 varchar(6), v2 varchar(2)) using parquet partitioned by (v2, c2);
 desc formatted char_part;
 
+alter table char_part change column c1 comment 'char comment';
+alter table char_part change column v1 comment 'varchar comment';
 alter table char_part add partition (v2='ke', c2='nt') location 'loc1';
 desc formatted char_part;
 

sql/core/src/test/resources/sql-tests/results/charvarchar.sql.out

@@ -556,6 +556,22 @@ Location [not included in comparison]/{warehouse_dir}/char_part
 Partition Provider  	Catalog
 
 
+-- !query
+alter table char_part change column c1 comment 'char comment'
+-- !query schema
+struct<>
+-- !query output
+
+
+
+-- !query
+alter table char_part change column v1 comment 'varchar comment'
+-- !query schema
+struct<>
+-- !query output
+
+
+
 -- !query
 alter table char_part add partition (v2='ke', c2='nt') location 'loc1'
 -- !query schema
@@ -569,8 +585,8 @@ desc formatted char_part
 -- !query schema
 struct<col_name:string,data_type:string,comment:string>
 -- !query output
-c1                  	char(5)             	                    
-v1                  	varchar(6)          	                    
+c1                  	char(5)             	char comment        
+v1                  	varchar(6)          	varchar comment     
 v2                  	varchar(2)          	                    
 c2                  	char(2)             	                    
 # Partition Information	                    	                    
@@ -612,8 +628,8 @@ desc formatted char_part
 -- !query schema
 struct<col_name:string,data_type:string,comment:string>
 -- !query output
-c1                  	char(5)             	                    
-v1                  	varchar(6)          	                    
+c1                  	char(5)             	char comment        
+v1                  	varchar(6)          	varchar comment     
 v2                  	varchar(2)          	                    
 c2                  	char(2)             	                    
 # Partition Information	                    	                    
@@ -647,8 +663,8 @@ desc formatted char_part
 -- !query schema
 struct<col_name:string,data_type:string,comment:string>
 -- !query output
-c1                  	char(5)             	                    
-v1                  	varchar(6)          	                    
+c1                  	char(5)             	char comment        
+v1                  	varchar(6)          	varchar comment     
 v2                  	varchar(2)          	                    
 c2                  	char(2)             	                    
 # Partition Information	                    	                    
@@ -682,8 +698,8 @@ desc formatted char_part
 -- !query schema
 struct<col_name:string,data_type:string,comment:string>
 -- !query output
-c1                  	char(5)             	                    
-v1                  	varchar(6)          	                    
+c1                  	char(5)             	char comment        
+v1                  	varchar(6)          	varchar comment     
 v2                  	varchar(2)          	                    
 c2                  	char(2)             	                    
 # Partition Information