[SPARK-2094][SQL] "Exactly once" semantics for DDL and command statements

## Related JIRA issues

- Main issue:

  - [SPARK-2094](https://issues.apache.org/jira/browse/SPARK-2094): Ensure exactly once semantics for DDL/Commands

- Issues resolved as dependencies:

  - [SPARK-2081](https://issues.apache.org/jira/browse/SPARK-2081): Undefine output() from the abstract class Command and implement it in concrete subclasses
  - [SPARK-2128](https://issues.apache.org/jira/browse/SPARK-2128): No plan for DESCRIBE
  - [SPARK-1852](https://issues.apache.org/jira/browse/SPARK-1852): SparkSQL Queries with Sorts run before the user asks them to

- Other related issue:

  - [SPARK-2129](https://issues.apache.org/jira/browse/SPARK-2129): NPE thrown while lookup a view

    Two test cases, `join_view` and `mergejoin_mixed`, within the `HiveCompatibilitySuite` are removed from the whitelist to workaround this issue.

## PR Overview

This PR defines physical plans for DDL statements and commands and wraps their side effects in a lazy field `PhysicalCommand.sideEffectResult`, so that they are executed eagerly and exactly once.  Also, as a positive side effect, now DDL statements and commands can be turned into proper `SchemaRDD`s and let user query the execution results.

This PR defines schemas for the following DDL/commands:

- EXPLAIN command

  - `plan`: String, the plan explanation

- SET command

  - `key`: String, the key(s) of the propert(y/ies) being set or queried
  - `value`: String, the value(s) of the propert(y/ies) being queried

- Other Hive native command

  - `result`: String, execution result returned by Hive

  **NOTE**: We should refine schemas for different native commands by defining physical plans for them in the future.

## Examples

### EXPLAIN command

Take the "EXPLAIN" command as an example, we first execute the command and obtain a `SchemaRDD` at the same time, then query the `plan` field with the schema DSL:

```
scala> loadTestTable("src")
...

scala> val q0 = hql("EXPLAIN SELECT key, COUNT(*) FROM src GROUP BY key")
...
q0: org.apache.spark.sql.SchemaRDD =
SchemaRDD[0] at RDD at SchemaRDD.scala:98
== Query Plan ==
ExplainCommandPhysical [plan#11:0]
 Aggregate false, [key#4], [key#4,SUM(PartialCount#6L) AS c_1#2L]
  Exchange (HashPartitioning [key#4:0], 200)
   Exchange (HashPartitioning [key#4:0], 200)
    Aggregate true, [key#4], [key#4,COUNT(1) AS PartialCount#6L]
     HiveTableScan [key#4], (MetastoreRelation default, src, None), None

scala> q0.select('plan).collect()
...
[ExplainCommandPhysical [plan#24:0]
 Aggregate false, [key#17], [key#17,SUM(PartialCount#19L) AS c_1#2L]
  Exchange (HashPartitioning [key#17:0], 200)
   Exchange (HashPartitioning [key#17:0], 200)
    Aggregate true, [key#17], [key#17,COUNT(1) AS PartialCount#19L]
     HiveTableScan [key#17], (MetastoreRelation default, src, None), None]

scala>
```

### SET command

In this example we query all the properties set in `SQLConf`, register the result as a table, and then query the table with HiveQL:

```
scala> val q1 = hql("SET")
...
q1: org.apache.spark.sql.SchemaRDD =
SchemaRDD[7] at RDD at SchemaRDD.scala:98
== Query Plan ==
<SET command: executed by Hive, and noted by SQLContext>

scala> q1.registerAsTable("properties")

scala> hql("SELECT key, value FROM properties ORDER BY key LIMIT 10").foreach(println)
...
== Query Plan ==
TakeOrdered 10, [key#51:0 ASC]
 Project [key#51:0,value#52:1]
  SetCommandPhysical None, None, [key#55:0,value#56:1]), which has no missing parents
14/06/12 12:19:27 INFO scheduler.DAGScheduler: Submitting 1 missing tasks from Stage 5 (SchemaRDD[21] at RDD at SchemaRDD.scala:98
== Query Plan ==
TakeOrdered 10, [key#51:0 ASC]
 Project [key#51:0,value#52:1]
  SetCommandPhysical None, None, [key#55:0,value#56:1])
...
[datanucleus.autoCreateSchema,true]
[datanucleus.autoStartMechanismMode,checked]
[datanucleus.cache.level2,false]
[datanucleus.cache.level2.type,none]
[datanucleus.connectionPoolingType,BONECP]
[datanucleus.fixedDatastore,false]
[datanucleus.identifierFactory,datanucleus1]
[datanucleus.plugin.pluginRegistryBundleCheck,LOG]
[datanucleus.rdbms.useLegacyNativeValueStrategy,true]
[datanucleus.storeManagerType,rdbms]

scala>
```

### "Exactly once" semantics

At last, an example of the "exactly once" semantics:

```
scala> val q2 = hql("CREATE TABLE t1(key INT, value STRING)")
...
q2: org.apache.spark.sql.SchemaRDD =
SchemaRDD[28] at RDD at SchemaRDD.scala:98
== Query Plan ==
<Native command: executed by Hive>

scala> table("t1")
...
res9: org.apache.spark.sql.SchemaRDD =
SchemaRDD[32] at RDD at SchemaRDD.scala:98
== Query Plan ==
HiveTableScan [key#58,value#59], (MetastoreRelation default, t1, None), None

scala> q2.collect()
...
res10: Array[org.apache.spark.sql.Row] = Array([])

scala>
```

As we can see, the "CREATE TABLE" command is executed eagerly right after the `SchemaRDD` is created, and referencing the `SchemaRDD` again won't trigger a duplicated execution.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1071 from liancheng/exactlyOnceCommand and squashes the following commits:

d005b03 [Cheng Lian] Made "SET key=value" returns the newly set key value pair
f6c7715 [Cheng Lian] Added test cases for DDL/command statement RDDs
1d00937 [Cheng Lian] Makes SchemaRDD DSLs work for DDL/command statement RDDs
5c7e680 [Cheng Lian] Bug fix: wrong type used in pattern matching
48aa2e5 [Cheng Lian] Refined SQLContext.emptyResult as an empty RDD[Row]
cc64f32 [Cheng Lian] Renamed physical plan classes for DDL/commands
74789c1 [Cheng Lian] Fixed failing test cases
0ad343a [Cheng Lian] Added physical plan for DDL and commands to ensure the "exactly once" semantics

7 files changed, 86 insertions, 80 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala b/sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala
index b6a2f1b9d1..378ff54531 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala
@@ -31,7 +31,7 @@ import org.apache.spark.sql.catalyst.{ScalaReflection, dsl}
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.types._
 import org.apache.spark.sql.catalyst.optimizer.Optimizer
-import org.apache.spark.sql.catalyst.plans.logical.{SetCommand, LogicalPlan}
+import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.RuleExecutor
 
 import org.apache.spark.sql.columnar.InMemoryRelation
@@ -147,14 +147,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
    *
    * @group userf
    */
-  def sql(sqlText: String): SchemaRDD = {
-    val result = new SchemaRDD(this, parseSql(sqlText))
-    // We force query optimization to happen right away instead of letting it happen lazily like
-    // when using the query DSL.  This is so DDL commands behave as expected.  This is only
-    // generates the RDD lineage for DML queries, but do not perform any execution.
-    result.queryExecution.toRdd
-    result
-  }
+  def sql(sqlText: String): SchemaRDD = new SchemaRDD(this, parseSql(sqlText))
 
   /** Returns the specified table as a SchemaRDD */
   def table(tableName: String): SchemaRDD =
@@ -259,8 +252,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
   protected[sql] val planner = new SparkPlanner
 
   @transient
-  protected[sql] lazy val emptyResult =
-    sparkContext.parallelize(Seq(new GenericRow(Array[Any]()): Row), 1)
+  protected[sql] lazy val emptyResult = sparkContext.parallelize(Seq.empty[Row], 1)
 
   /**
    * Prepares a planned SparkPlan for execution by binding references to specific ordinals, and
@@ -280,22 +272,6 @@ class SQLContext(@transient val sparkContext: SparkContext)
   protected abstract class QueryExecution {
     def logical: LogicalPlan
 
-    def eagerlyProcess(plan: LogicalPlan): RDD[Row] = plan match {
-      case SetCommand(key, value) =>
-        // Only this case needs to be executed eagerly. The other cases will
-        // be taken care of when the actual results are being extracted.
-        // In the case of HiveContext, sqlConf is overridden to also pass the
-        // pair into its HiveConf.
-        if (key.isDefined && value.isDefined) {
-          set(key.get, value.get)
-        }
-        // It doesn't matter what we return here, since this is only used
-        // to force the evaluation to happen eagerly.  To query the results,
-        // one must use SchemaRDD operations to extract them.
-        emptyResult
-      case _ => executedPlan.execute()
-    }
-
     lazy val analyzed = analyzer(logical)
     lazy val optimizedPlan = optimizer(analyzed)
     // TODO: Don't just pick the first one...
@@ -303,12 +279,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
     lazy val executedPlan: SparkPlan = prepareForExecution(sparkPlan)
 
     /** Internal version of the RDD. Avoids copies and has no schema */
-    lazy val toRdd: RDD[Row] = {
-      logical match {
-        case s: SetCommand => eagerlyProcess(s)
-        case _ => executedPlan.execute()
-      }
-    }
+    lazy val toRdd: RDD[Row] = executedPlan.execute()
 
     protected def stringOrError[A](f: => A): String =
       try f.toString catch { case e: Throwable => e.toString }
@@ -330,7 +301,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
    * TODO: We only support primitive types, add support for nested types.
    */
   private[sql] def inferSchema(rdd: RDD[Map[String, _]]): SchemaRDD = {
-    val schema = rdd.first.map { case (fieldName, obj) =>
+    val schema = rdd.first().map { case (fieldName, obj) =>
       val dataType = obj.getClass match {
         case c: Class[_] if c == classOf[java.lang.String] => StringType
         case c: Class[_] if c == classOf[java.lang.Integer] => IntegerType
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/SchemaRDD.scala b/sql/core/src/main/scala/org/apache/spark/sql/SchemaRDD.scala
index 7ad8edf5a5..821ac850ac 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/SchemaRDD.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/SchemaRDD.scala
@@ -97,7 +97,7 @@ import java.util.{Map => JMap}
 @AlphaComponent
 class SchemaRDD(
     @transient val sqlContext: SQLContext,
-    @transient protected[spark] val logicalPlan: LogicalPlan)
+    @transient val baseLogicalPlan: LogicalPlan)
   extends RDD[Row](sqlContext.sparkContext, Nil) with SchemaRDDLike {
 
   def baseSchemaRDD = this
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/SchemaRDDLike.scala b/sql/core/src/main/scala/org/apache/spark/sql/SchemaRDDLike.scala
index 3a895e15a4..656be965a8 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/SchemaRDDLike.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/SchemaRDDLike.scala
@@ -20,13 +20,14 @@ package org.apache.spark.sql
 import org.apache.spark.annotation.{DeveloperApi, Experimental}
 import org.apache.spark.sql.catalyst.analysis.UnresolvedRelation
 import org.apache.spark.sql.catalyst.plans.logical._
+import org.apache.spark.sql.execution.SparkLogicalPlan
 
 /**
  * Contains functions that are shared between all SchemaRDD types (i.e., Scala, Java)
  */
 private[sql] trait SchemaRDDLike {
   @transient val sqlContext: SQLContext
-  @transient protected[spark] val logicalPlan: LogicalPlan
+  @transient val baseLogicalPlan: LogicalPlan
 
   private[sql] def baseSchemaRDD: SchemaRDD
 
@@ -48,7 +49,17 @@ private[sql] trait SchemaRDDLike {
    */
   @transient
   @DeveloperApi
-  lazy val queryExecution = sqlContext.executePlan(logicalPlan)
+  lazy val queryExecution = sqlContext.executePlan(baseLogicalPlan)
+
+  @transient protected[spark] val logicalPlan: LogicalPlan = baseLogicalPlan match {
+    // For various commands (like DDL) and queries with side effects, we force query optimization to
+    // happen right away to let these side effects take place eagerly.
+    case _: Command | _: InsertIntoTable | _: InsertIntoCreatedTable | _: WriteToFile =>
+      queryExecution.toRdd
+      SparkLogicalPlan(queryExecution.executedPlan)
+    case _ =>
+      baseLogicalPlan
+  }
 
   override def toString =
     s"""${super.toString}
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/api/java/JavaSchemaRDD.scala b/sql/core/src/main/scala/org/apache/spark/sql/api/java/JavaSchemaRDD.scala
index 22f57b758d..aff6ffe9f3 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/api/java/JavaSchemaRDD.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/api/java/JavaSchemaRDD.scala
@@ -37,7 +37,7 @@ import org.apache.spark.storage.StorageLevel
  */
 class JavaSchemaRDD(
      @transient val sqlContext: SQLContext,
-     @transient protected[spark] val logicalPlan: LogicalPlan)
+     @transient val baseLogicalPlan: LogicalPlan)
   extends JavaRDDLike[Row, JavaRDD[Row]]
   with SchemaRDDLike {
 
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
index a657911afe..2233216a6e 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark.sql.execution
 
-import org.apache.spark.sql.{SQLConf, SQLContext, execution}
+import org.apache.spark.sql.{SQLContext, execution}
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.planning._
 import org.apache.spark.sql.catalyst.plans._
@@ -157,7 +157,7 @@ private[sql] abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
         InsertIntoParquetTable(relation, planLater(child), overwrite=true)(sparkContext) :: Nil
       case logical.InsertIntoTable(table: ParquetRelation, partition, child, overwrite) =>
         InsertIntoParquetTable(table, planLater(child), overwrite)(sparkContext) :: Nil
-      case PhysicalOperation(projectList, filters: Seq[Expression], relation: ParquetRelation) => {
+      case PhysicalOperation(projectList, filters: Seq[Expression], relation: ParquetRelation) =>
         val prunePushedDownFilters =
           if (sparkContext.conf.getBoolean(ParquetFilters.PARQUET_FILTER_PUSHDOWN_ENABLED, true)) {
             (filters: Seq[Expression]) => {
@@ -186,7 +186,6 @@ private[sql] abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
           filters,
           prunePushedDownFilters,
           ParquetTableScan(_, relation, filters)(sparkContext)) :: Nil
-      }
 
       case _ => Nil
     }
@@ -250,12 +249,12 @@ private[sql] abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
   case class CommandStrategy(context: SQLContext) extends Strategy {
     def apply(plan: LogicalPlan): Seq[SparkPlan] = plan match {
       case logical.SetCommand(key, value) =>
-        Seq(execution.SetCommandPhysical(key, value, plan.output)(context))
+        Seq(execution.SetCommand(key, value, plan.output)(context))
       case logical.ExplainCommand(child) =>
         val executedPlan = context.executePlan(child).executedPlan
-        Seq(execution.ExplainCommandPhysical(executedPlan, plan.output)(context))
+        Seq(execution.ExplainCommand(executedPlan, plan.output)(context))
       case logical.CacheCommand(tableName, cache) =>
-        Seq(execution.CacheCommandPhysical(tableName, cache)(context))
+        Seq(execution.CacheCommand(tableName, cache)(context))
       case _ => Nil
     }
   }
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/commands.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/commands.scala
index be26d19e66..0377290af5 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/commands.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/commands.scala
@@ -22,45 +22,69 @@ import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.{SQLContext, Row}
 import org.apache.spark.sql.catalyst.expressions.{GenericRow, Attribute}
 
+trait Command {
+  /**
+   * A concrete command should override this lazy field to wrap up any side effects caused by the
+   * command or any other computation that should be evaluated exactly once. The value of this field
+   * can be used as the contents of the corresponding RDD generated from the physical plan of this
+   * command.
+   *
+   * The `execute()` method of all the physical command classes should reference `sideEffectResult`
+   * so that the command can be executed eagerly right after the command query is created.
+   */
+  protected[sql] lazy val sideEffectResult: Seq[Any] = Seq.empty[Any]
+}
+
 /**
  * :: DeveloperApi ::
  */
 @DeveloperApi
-case class SetCommandPhysical(key: Option[String], value: Option[String], output: Seq[Attribute])
-                             (@transient context: SQLContext) extends LeafNode {
-  def execute(): RDD[Row] = (key, value) match {
-    // Set value for key k; the action itself would
-    // have been performed in QueryExecution eagerly.
-    case (Some(k), Some(v)) => context.emptyResult
+case class SetCommand(
+    key: Option[String], value: Option[String], output: Seq[Attribute])(
+    @transient context: SQLContext)
+  extends LeafNode with Command {
+
+  override protected[sql] lazy val sideEffectResult: Seq[(String, String)] = (key, value) match {
+    // Set value for key k.
+    case (Some(k), Some(v)) =>
+      context.set(k, v)
+      Array(k -> v)
+
     // Query the value bound to key k.
-    case (Some(k), None) =>
-      val resultString = context.getOption(k) match {
-        case Some(v) => s"$k=$v"
-        case None => s"$k is undefined"
-      }
-      context.sparkContext.parallelize(Seq(new GenericRow(Array[Any](resultString))), 1)
+    case (Some(k), _) =>
+      Array(k -> context.getOption(k).getOrElse("<undefined>"))
+
     // Query all key-value pairs that are set in the SQLConf of the context.
     case (None, None) =>
-      val pairs = context.getAll
-      val rows = pairs.map { case (k, v) =>
-        new GenericRow(Array[Any](s"$k=$v"))
-      }.toSeq
-      // Assume config parameters can fit into one split (machine) ;)
-      context.sparkContext.parallelize(rows, 1)
-    // The only other case is invalid semantics and is impossible.
-    case _ => context.emptyResult
+      context.getAll
+
+    case _ =>
+      throw new IllegalArgumentException()
   }
+
+  def execute(): RDD[Row] = {
+    val rows = sideEffectResult.map { case (k, v) => new GenericRow(Array[Any](k, v)) }
+    context.sparkContext.parallelize(rows, 1)
+  }
+
+  override def otherCopyArgs = context :: Nil
 }
 
 /**
  * :: DeveloperApi ::
  */
 @DeveloperApi
-case class ExplainCommandPhysical(child: SparkPlan, output: Seq[Attribute])
-                                 (@transient context: SQLContext) extends UnaryNode {
+case class ExplainCommand(
+    child: SparkPlan, output: Seq[Attribute])(
+    @transient context: SQLContext)
+  extends UnaryNode with Command {
+
+  // Actually "EXPLAIN" command doesn't cause any side effect.
+  override protected[sql] lazy val sideEffectResult: Seq[String] = this.toString.split("\n")
+
   def execute(): RDD[Row] = {
-    val planString = new GenericRow(Array[Any](child.toString))
-    context.sparkContext.parallelize(Seq(planString))
+    val explanation = sideEffectResult.mkString("\n")
+    context.sparkContext.parallelize(Seq(new GenericRow(Array[Any](explanation))), 1)
   }
 
   override def otherCopyArgs = context :: Nil
@@ -70,19 +94,20 @@ case class ExplainCommandPhysical(child: SparkPlan, output: Seq[Attribute])
  * :: DeveloperApi ::
  */
 @DeveloperApi
-case class CacheCommandPhysical(tableName: String, doCache: Boolean)(@transient context: SQLContext)
-  extends LeafNode {
+case class CacheCommand(tableName: String, doCache: Boolean)(@transient context: SQLContext)
+  extends LeafNode with Command {
 
-  lazy val commandSideEffect = {
+  override protected[sql] lazy val sideEffectResult = {
     if (doCache) {
       context.cacheTable(tableName)
     } else {
       context.uncacheTable(tableName)
     }
+    Seq.empty[Any]
   }
 
   override def execute(): RDD[Row] = {
-    commandSideEffect
+    sideEffectResult
     context.emptyResult
   }
 
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
index c1fc99f077..e9360b0fc7 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
@@ -141,7 +141,7 @@ class SQLQuerySuite extends QueryTest {
       sql("SELECT AVG(a),b FROM largeAndSmallInts group by b"),
       Seq((2147483645.0,1),(2.0,2)))
   }
-  
+
   test("count") {
     checkAnswer(
       sql("SELECT COUNT(*) FROM testData2"),
@@ -332,7 +332,7 @@ class SQLQuerySuite extends QueryTest {
         (3, "C"),
         (4, "D")))
   }
-  
+
   test("system function upper()") {
     checkAnswer(
       sql("SELECT n,UPPER(l) FROM lowerCaseData"),
@@ -349,7 +349,7 @@ class SQLQuerySuite extends QueryTest {
         (2, "ABC"),
         (3, null)))
   }
-    
+
   test("system function lower()") {
     checkAnswer(
       sql("SELECT N,LOWER(L) FROM upperCaseData"),
@@ -382,25 +382,25 @@ class SQLQuerySuite extends QueryTest {
     sql(s"SET $testKey=$testVal")
     checkAnswer(
       sql("SET"),
-      Seq(Seq(s"$testKey=$testVal"))
+      Seq(Seq(testKey, testVal))
     )
 
     sql(s"SET ${testKey + testKey}=${testVal + testVal}")
     checkAnswer(
       sql("set"),
       Seq(
-        Seq(s"$testKey=$testVal"),
-        Seq(s"${testKey + testKey}=${testVal + testVal}"))
+        Seq(testKey, testVal),
+        Seq(testKey + testKey, testVal + testVal))
     )
 
     // "set key"
     checkAnswer(
       sql(s"SET $testKey"),
-      Seq(Seq(s"$testKey=$testVal"))
+      Seq(Seq(testKey, testVal))
     )
     checkAnswer(
       sql(s"SET $nonexistentKey"),
-      Seq(Seq(s"$nonexistentKey is undefined"))
+      Seq(Seq(nonexistentKey, "<undefined>"))
     )
     clear()
   }