[SPARK-13616][SQL] Let SQLBuilder convert logical plan without a project on top of it

JIRA: https://issues.apache.org/jira/browse/SPARK-13616

## What changes were proposed in this pull request?

It is possibly that a logical plan has been removed `Project` from the top of it. Or the plan doesn't has a top `Project` from the beginning because it is not necessary. Currently the `SQLBuilder` can't convert such plans back to SQL. This change is to add this feature.

## How was this patch tested?

A test is added to `LogicalPlanToSQLSuite`.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #11466 from viirya/sqlbuilder-notopselect.

sql/hive/src/main/scala/org/apache/spark/sql/hive/SQLBuilder.scala

@@ -53,7 +53,7 @@ class SQLBuilder(logicalPlan: LogicalPlan, sqlContext: SQLContext) extends Loggi
         case e => e
       }
 
-      val generatedSQL = toSQL(canonicalizedPlan)
+      val generatedSQL = toSQL(canonicalizedPlan, true)
       logDebug(
         s"""Built SQL query string successfully from given logical plan:
            |
@@ -78,6 +78,27 @@ class SQLBuilder(logicalPlan: LogicalPlan, sqlContext: SQLContext) extends Loggi
     }
   }
 
+  private def toSQL(node: LogicalPlan, topNode: Boolean): String = {
+    if (topNode) {
+      node match {
+        case d: Distinct => toSQL(node)
+        case p: Project => toSQL(node)
+        case a: Aggregate => toSQL(node)
+        case s: Sort => toSQL(node)
+        case r: RepartitionByExpression => toSQL(node)
+        case _ =>
+          build(
+            "SELECT",
+            node.output.map(_.sql).mkString(", "),
+            "FROM",
+            toSQL(node)
+          )
+      }
+    } else {
+      toSQL(node)
+    }
+  }
+
   private def toSQL(node: LogicalPlan): String = node match {
     case Distinct(p: Project) =>
       projectToSQL(p, isDistinct = true)

sql/hive/src/test/scala/org/apache/spark/sql/hive/LogicalPlanToSQLSuite.scala

@@ -19,6 +19,8 @@ package org.apache.spark.sql.hive
 
 import scala.util.control.NonFatal
 
+import org.apache.spark.sql.{DataFrame, SQLContext}
+import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.functions._
 import org.apache.spark.sql.test.SQLTestUtils
 
@@ -54,6 +56,33 @@ class LogicalPlanToSQLSuite extends SQLBuilderTest with SQLTestUtils {
     sql("DROP TABLE IF EXISTS t0")
   }
 
+  private def checkPlan(plan: LogicalPlan, sqlContext: SQLContext, expected: String): Unit = {
+    val convertedSQL = try new SQLBuilder(plan, sqlContext).toSQL catch {
+      case NonFatal(e) =>
+        fail(
+          s"""Cannot convert the following logical query plan back to SQL query string:
+             |
+             |# Original logical query plan:
+             |${plan.treeString}
+           """.stripMargin, e)
+    }
+
+    try {
+      checkAnswer(sql(convertedSQL), DataFrame(sqlContext, plan))
+    } catch { case cause: Throwable =>
+      fail(
+        s"""Failed to execute converted SQL string or got wrong answer:
+           |
+           |# Converted SQL query string:
+           |$convertedSQL
+           |
+           |# Original logical query plan:
+           |${plan.treeString}
+         """.stripMargin,
+        cause)
+    }
+  }
+
   private def checkHiveQl(hiveQl: String): Unit = {
     val df = sql(hiveQl)
 
@@ -157,6 +186,18 @@ class LogicalPlanToSQLSuite extends SQLBuilderTest with SQLTestUtils {
       "SELECT x.key, COUNT(*) FROM parquet_t1 x JOIN parquet_t1 y ON x.key = y.key group by x.key")
   }
 
+  test("join plan") {
+    val expectedSql = "SELECT x.key FROM parquet_t1 x JOIN parquet_t1 y ON x.key = y.key"
+
+    val df1 = sqlContext.table("parquet_t1").as("x")
+    val df2 = sqlContext.table("parquet_t1").as("y")
+    val joinPlan = df1.join(df2).queryExecution.analyzed
+
+    // Make sure we have a plain Join operator without Project on top of it.
+    assert(joinPlan.isInstanceOf[Join])
+    checkPlan(joinPlan, sqlContext, expectedSql)
+  }
+
   test("case") {
     checkHiveQl("SELECT CASE WHEN id % 2 > 0 THEN 0 WHEN id % 2 = 0 THEN 1 END FROM parquet_t0")
   }