Github user liancheng commented on a diff in the pull request:
https://github.com/apache/spark/pull/1071#discussion_r13734016
--- Diff:
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 PhysicalCommand {
+ /**
+ * 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 `sideEffect` 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]
--- End diff --
After some thought I think it's not only about naming, the semantics is
wrong: en `RDD[Row]` with an empty row indicates that the schema of the result
has *no* fields, while an empty `RDD[Row]` can be fit schemas with any number
of fields. And, for `SELECT 1`, shouldn't it be an `RDD[Row]` with a single row
containing a `1`?
There does exist a `singleRowRdd` in `SparkStrategies`, used to translate
`logical.NoRelation`.
Currently `emptyResult` is only used in `execution.CacheCommand` and
`execution.NativeCommand`. So I'll just fix `emptyResult` definition and see if
all test suites pass.
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