Improve custom schema.

external/docker-integration-tests/src/test/scala/org/apache/spark/sql/jdbc/OracleIntegrationSuite.scala

@@ -217,10 +217,9 @@ class OracleIntegrationSuite extends DockerJDBCIntegrationSuite with SharedSQLCo
 
     // custom schema can read data
     val schema = StructType(Seq(
-      StructField("ID", DecimalType(DecimalType.MAX_PRECISION, 0), true,
-        new MetadataBuilder().putString("name", "ID").build()),
-      StructField("N1", IntegerType, true, new MetadataBuilder().putString("name", "N1").build()),
-      StructField("N2", BooleanType, true, new MetadataBuilder().putString("name", "N2").build())))
+      StructField("ID", DecimalType(DecimalType.MAX_PRECISION, 0)),
+      StructField("N1", IntegerType, true),
+      StructField("N2", BooleanType, true)))
 
     val dfRead = spark.read.schema(schema).jdbc(jdbcUrl, "custom_column_types", new Properties())
     val rows = dfRead.collect()

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCRDD.scala

@@ -82,8 +82,8 @@ object JDBCRDD extends Logging {
    * @return A Catalyst schema corresponding to columns in the given order.
    */
   private def pruneSchema(schema: StructType, columns: Array[String]): StructType = {
-    val fieldMap = Map(schema.fields.map(x => x.metadata.getString("name") -> x): _*)
-    new StructType(columns.map(name => fieldMap(name)))
+     val fieldMap = Map(schema.fields.map(x => x.name -> x): _*)
+     new StructType(columns.map(name => fieldMap(name)))
   }
 
   /**