## 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
ac96d9657c