Github user yjshen commented on the pull request:
https://github.com/apache/spark/pull/7986#issuecomment-128231658
The reason for the bug:
Give `EXPLAIN CREATE TABLE t1 SELECT * FROM exist_table` as an example.
`ExplainCommand(CreateTableAsSelect)` is used internally to run the command:
```scala
val queryExecution = sqlContext.executePlan(logicalPlan/*It is
CreateTableAsSelect in this case*/)
val outputString = if (extended) queryExecution.toString else
queryExecution.simpleString
```
After analysing phase, createTableAsSelect was converted to another
RunnableCommand, which is a leaf operator by default:
``` scala
private[sql] trait RunnableCommand extends LogicalPlan with logical.Command
{
override def children: Seq[LogicalPlan] = Seq.empty
```
Therefore, no further optimisation or planning is feasible in this case.
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