[SQL] Implement Describe Table for SQLContext

[yanboliang]

Initial code snippet for Describe Table command

1. SQL Parser and generate logical plan. Add "DESCRIBE [FORMATTED] [db_name.]table_name" command parser in DDLParser and generate the same logical plan for both SQLContext and HiveContext.
   (note: HiveContext also support "DESCRIBE [FORMATTED] [db_name.]table_name PARTITION partition_column_name" and "DESCRIBE [FORMATTED] [db_name.]table_name column_name" is implement by Hive native command)
2. Implement DescribeCommand which invoke by RunnableCommand.
3. For SQLContext the code is clearly structured, but for HiveContext the output of describe command need to stay the same Hive. So For HiveContext, we still transfer logical command to DescribeHiveTableCommand which had been implement in HiveStrategies.HiveCommandStrategy.

[yanboliang]

https://issues.apache.org/jira/browse/SPARK-5324

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[yanboliang]

Move Describe command parser to DDLParser which can implement do not throw error when parse exception. So that some HiveContext particular describe command can be parse by fallback Hive parser and run by Hive native command.

[lianhuiwang]

firstly,we need to rename title of PR to [SPARK-5324][SQL] Implement Describe Table for SQLContext. i find that code of this PR is not the latest code. i donot agree that move DescribeCommand to a class of SparkPlan. it keep to be a class of LogicalPlan.
in Hive Context, it have implemented DescribeHiveTableCommand of SparkPlan. so in SQL Context, we just need to implement a SparkPlan like DescribeHiveTableCommand of Hive Context. and in CommandStrategy of SparkStrategies will translate Describe's LogicalPlan to SparkPlan.

[rxin]

Thanks for submitting the pull request. Are these two PRs working on the same thing? #4227 Would be great if you two can chime in on each other's PR.

[OopsOutOfMemory]

hi, @yanbohappy, I've already worked in this.
For your PR, I took a look at it. But I think it's a little hacky to me. Would you like to review mine?

[OopsOutOfMemory]

IntegerType and StringType ? should be int, string

[OopsOutOfMemory]

hi, @yanbohappy Thanks for working on this. But by the way, this JIRA(SPARK-5324) Results of describe can't be queried is mainly focus on make describe command can be query like a table but not Add a describe command in sqlContext, shall we make this PR focus on it's own JIRA issue? And you have no more test suites to demonstrate bug fixing. Would you mind close this PR, and if you have some good advices for add describe table you can refer to SPARK-5135 #4227 , and comment on my PR. I'd be very pleasure : )

[yanboliang]

@lianhuiwang In this PR #3948, CommandStrategy had been removed and command had been refactor.

[yanboliang]

@OopsOutOfMemory Since you have go deep into this issue and I agree your PR is more mature. So close this one.

[rxin]

@yanbohappy thanks for the discussion.

[OopsOutOfMemory]

@yanbohappy thanks for the discussion and the work efforts : )