Fix map single table (#319)

README.md

@@ -31,12 +31,33 @@ Uses as below
 import org.apache.spark.sql.TiContext
 val ti = new TiContext(spark) 
 
-// Mapping all TiDB tables from database tpch as Spark SQL tables
+// Map all TiDB tables from database tpch as Spark SQL tables
 ti.tidbMapDatabase("tpch")
 
 spark.sql("select count(*) from lineitem").show
 ```
 
+## Metadata loading
+If you are using spark-shell, you need to manually load schema information as decribed above.
+
+If you have too many tables, you might choose to disable histogram preparison and loading will be faster.
+
+```
+ti.tidbMapDatabase("tpch", autoLoadStatistics = true)
+```
+
+If you have two tables with same name in different databases, you might choose to append database name as prefix for table name:
+
+```
+ti.tidbMapDatabase("tpch", dbNameAsPrefix = true)
+```
+
+If you have too many tables and use only some of them, to speed up meta loading process, you might manually load only tables you use:
+
+```
+ti.tidbTable("tpch", "lineitem")
+```
+
 ## Current Version
 ```
 ti.version

core/src/main/scala/org/apache/spark/sql/TiContext.scala

@@ -129,13 +129,15 @@ class TiContext(val session: SparkSession) extends Serializable with Logging {
     }
   }
 
-  def tidbTable(dbName: String, tableName: String): DataFrame = {
+  // tidbMapTable does not do any check any meta information
+  // it just register table for later use
+  def tidbMapTable(dbName: String, tableName: String): Unit = {
     val tiRelation = new TiDBRelation(
       tiSession,
       new TiTableReference(dbName, tableName),
       meta
     )(sqlContext)
-    sqlContext.baseRelationToDataFrame(tiRelation)
+    sqlContext.baseRelationToDataFrame(tiRelation).createTempView(tableName)
   }
 
   def tidbMapDatabase(dbName: String,