Improve test.

Author: wangyum

docs/sql-programming-guide.md

@@ -1333,7 +1333,7 @@ the following case-insensitive options:
   <tr>
     <td><code>customSchema</code></td>
     <td>
-     The custom schema to use for reading data from JDBC connectors. For example, <code>"id DECIMAL(38, 0), name STRING"</code>. You can also specify partial fields, others use default values. For example, <code>"id DECIMAL(38, 0)"</code>. The column names should be identical to the corresponding column names of JDBC table. Users can specify the corresponding data types of Spark SQL instead of using the defaults. This option applies only to reading.
+     The custom schema to use for reading data from JDBC connectors. For example, <code>"id DECIMAL(38, 0), name STRING"</code>. You can also specify partial fields, and the others use the default type mapping. For example, <code>"id DECIMAL(38, 0)"</code>. The column names should be identical to the corresponding column names of JDBC table. Users can specify the corresponding data types of Spark SQL instead of using the defaults. This option applies only to reading.
     </td>
   </tr>
 </table>

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

@@ -784,10 +784,8 @@ object JdbcUtils extends Logging {
       // This is resolved by names, use the custom filed dataType to replace the default dateType.
       val newSchema = tableSchema.map { col =>
         userSchema.find(f => nameEquality(f.name, col.name)) match {
-          case Some(c) =>
-            col.copy(dataType = c.dataType, nullable = c.nullable)
-          case None =>
-            col
+          case Some(c) => col.copy(dataType = c.dataType, metadata = Metadata.empty)
+          case None => col
         }
       }
       StructType(newSchema)

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

@@ -26,6 +26,7 @@ import org.scalatest.{BeforeAndAfter, PrivateMethodTester}
 
 import org.apache.spark.{SparkException, SparkFunSuite}
 import org.apache.spark.sql.{AnalysisException, DataFrame, Row}
+import org.apache.spark.sql.catalyst.parser.CatalystSqlParser
 import org.apache.spark.sql.catalyst.util.CaseInsensitiveMap
 import org.apache.spark.sql.execution.DataSourceScanExec
 import org.apache.spark.sql.execution.command.ExplainCommand
@@ -970,36 +971,28 @@ class JDBCSuite extends SparkFunSuite
 
   test("jdbc API support custom schema") {
     val parts = Array[String]("THEID < 2", "THEID >= 2")
+    val customSchema = "NAME STRING, THEID INT"
     val props = new Properties()
-    props.put("customSchema", "name STRING, THEID BIGINT")
-    val schema = StructType(Seq(
-      StructField("NAME", StringType, true), StructField("THEID", LongType, true)))
+    props.put("customSchema", customSchema)
     val df = spark.read.jdbc(urlWithUserAndPass, "TEST.PEOPLE", parts, props)
     assert(df.schema.size === 2)
-    df.schema.zip(schema).foreach {
-      case (c, v) =>
-        assert(c.dataType === v.dataType)
-    }
+    assert(df.schema === CatalystSqlParser.parseTableSchema(customSchema))
     assert(df.count() === 3)
   }
 
   test("jdbc API custom schema DDL-like strings.") {
     withTempView("people_view") {
+      val customSchema = "NAME STRING, THEID INT"
       sql(
         s"""
            |CREATE TEMPORARY VIEW people_view
            |USING org.apache.spark.sql.jdbc
            |OPTIONS (uRl '$url', DbTaBlE 'TEST.PEOPLE', User 'testUser', PassWord 'testPass',
-           |customSchema 'NAME STRING, THEID INT')
+           |customSchema '$customSchema')
         """.stripMargin.replaceAll("\n", " "))
-      val schema = StructType(
-        Seq(StructField("NAME", StringType, true), StructField("THEID", IntegerType, true)))
       val df = sql("select * from people_view")
-      assert(df.schema.size === 2)
-      df.schema.zip(schema).foreach {
-        case (c, v) =>
-          assert(c.dataType === v.dataType)
-      }
+      assert(df.schema.length === 2)
+      assert(df.schema === CatalystSqlParser.parseTableSchema(customSchema))
       assert(df.count() === 3)
     }
   }