Fixed review

Author: zaleslaw

dataframe-jdbc/api/dataframe-jdbc.api

@@ -109,6 +109,7 @@ public abstract class org/jetbrains/kotlinx/dataframe/io/db/DbType {
 	public fun getDefaultFetchSize ()I
 	public fun getDefaultQueryTimeout ()Ljava/lang/Integer;
 	public abstract fun getDriverClassName ()Ljava/lang/String;
+	public fun getTableColumnsMetadata (Ljava/sql/ResultSet;)Ljava/util/List;
 	public fun getTableTypes ()Ljava/util/List;
 	public abstract fun isSystemTable (Lorg/jetbrains/kotlinx/dataframe/io/db/TableMetadata;)Z
 	public fun makeCommonSqlToKTypeMapping (Lorg/jetbrains/kotlinx/dataframe/io/db/TableColumnMetadata;)Lkotlin/reflect/KType;

dataframe-jdbc/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/db/DbType.kt

@@ -412,9 +412,9 @@ public abstract class DbType(public val dbTypeInJdbcUrl: String) {
      * where `DatabaseMetaData.getColumns` is known to be slow).
      *
      * @param resultSet The [ResultSet] containing query results.
-     * @return A mutable list of [TableColumnMetadata] objects.
+     * @return A list of [TableColumnMetadata] objects.
      */
-    public open fun getTableColumnsMetadata(resultSet: ResultSet): MutableList<TableColumnMetadata> {
+    public open fun getTableColumnsMetadata(resultSet: ResultSet): List<TableColumnMetadata> {
         val rsMetaData = resultSet.metaData
         val connection = resultSet.statement.connection
         val dbMetaData = connection.metaData
@@ -446,8 +446,9 @@ public abstract class DbType(public val dbTypeInJdbcUrl: String) {
             } catch (_: Exception) {
                 // Some drivers may throw for unsupported features
                 // In that case, fallback to DatabaseMetaData
-                val cols = dbMetaData.getColumns(catalog, schema, tableName, columnName)
-                if (cols.next()) cols.getString("IS_NULLABLE") == "YES" else true
+                dbMetaData.getColumns(catalog, schema, tableName, columnName).use { cols ->
+                    if (cols.next()) !cols.getString("IS_NULLABLE").equals("NO", ignoreCase = true) else true
+                }
             }
 
             val columnType = rsMetaData.getColumnTypeName(index)

dataframe-jdbc/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/readJdbc.kt

@@ -852,7 +852,7 @@ private fun readTableAsDataFrame(
 }
 
 internal fun getTableColumnsMetadata(resultSet: ResultSet, dbType: DbType): MutableList<TableColumnMetadata> =
-    dbType.getTableColumnsMetadata(resultSet)
+    dbType.getTableColumnsMetadata(resultSet).toMutableList()
 
 /**
  * Fetches and converts data from a ResultSet into a mutable map.