[LIVY-650][THRIFT] Remove schema from ResultSet

## What changes were proposed in this pull request?

The class `ResultSet` is serialized and sent over the wire. Currently this class contains a JSON string representation of the spark schema, which is never used. Hence, the PR removes it in order to avoid serializing it uselessly.

## How was this patch tested?

existing UTs

Author: Marco Gaido <[email protected]>

Closes apache#213 from mgaido91/LIVY-650.

thriftserver/session/src/main/java/org/apache/livy/thriftserver/session/FetchResultJob.java

@@ -49,7 +49,7 @@ public ResultSet call(JobContext ctx) {
     StatementState st = session.findStatement(statementId);
     Iterator<Row> iter = st.iter;
 
-    ResultSet rs = new ResultSet(st.types, st.schema);
+    ResultSet rs = new ResultSet(st.types);
     int count = 0;
     while (iter.hasNext() && count < maxRows) {
       Row row = iter.next();

thriftserver/session/src/main/java/org/apache/livy/thriftserver/session/ResultSet.java

@@ -37,16 +37,13 @@
  */
 public class ResultSet {
 
-  private final String schema;
   private final ColumnBuffer[] columns;
 
   public ResultSet() {
-    this.schema = null;
     this.columns = null;
   }
 
-  public ResultSet(DataType[] types, String schema) {
-    this.schema = schema;
+  public ResultSet(DataType[] types) {
     this.columns = new ColumnBuffer[types.length];
     for (int i = 0; i < columns.length; i++) {
       columns[i] = new ColumnBuffer(types[i]);
@@ -69,10 +66,6 @@ public void addRow(Object[] fields) {
     }
   }
 
-  public String getSchema() {
-    return schema;
-  }
-
   public ColumnBuffer[] getColumns() {
     return columns;
   }

thriftserver/session/src/main/java/org/apache/livy/thriftserver/session/SqlJob.java

@@ -18,9 +18,7 @@
 package org.apache.livy.thriftserver.session;
 
 import java.util.Iterator;
-import java.util.List;
 
-import org.apache.spark.rdd.RDD;
 import org.apache.spark.sql.Dataset;
 import org.apache.spark.sql.Row;
 import org.apache.spark.sql.SparkSession;
@@ -88,5 +86,4 @@ private void executeSql(JobContext ctx) throws Exception {
     // has been executed.
     session.registerStatement(statementId, schema, iter);
   }
-
 }

thriftserver/session/src/main/java/org/apache/livy/thriftserver/session/StatementState.java

@@ -36,5 +36,4 @@ class StatementState {
     this.iter = iter;
     this.types = SparkUtils.translateSchema(schema);
   }
-
 }

thriftserver/session/src/main/java/org/apache/livy/thriftserver/session/ThriftSessionState.java

@@ -157,5 +157,4 @@ private NoSuchElementException catalogJobNotFound(String jobId) {
     return new NoSuchElementException(
         String.format("Catalog job %s not found in session %s.", jobId, sessionId));
   }
-
 }

thriftserver/session/src/test/java/org/apache/livy/thriftserver/session/ColumnBufferTest.java

@@ -79,7 +79,7 @@ public void testColumnBuffer() throws Exception {
 
       ds.write().format("parquet").saveAsTable("types_test");
 
-      ResultSet rs = new ResultSet(SparkUtils.translateSchema(ds.schema()), ds.schema().json());
+      ResultSet rs = new ResultSet(SparkUtils.translateSchema(ds.schema()));
       for (Row r : spark.table("types_test").collectAsList()) {
         Object[] cols = new Object[r.length()];
         for (int i = 0; i < cols.length; i++) {