[FLINK-3192] Add explain support to print ast and sql physical execution.

[gallenvara]

Table API doesn't support sql-explanation now. Add the explain support to print ast (abstract syntax tree) and the physical execution of sql.

[fhueske]

Why do we need this dependency?

[fhueske]

Hi @gallenvara, thanks for this PR!
Explain is a very nice feature for the Table API (and later on for the SQL interface).

IMO, it would be better to reuse the existing JSON plan and generate the HIVE visualization locally inside of the Table API. That would avoid to replicate quite a bit of functionality just for the sake of a different visualization. Would that work as well?

In general, it is a good idea to discuss changes that modify user-facing APIs and touch several components/modules before starting to code as pointed out in our contribution guidelines.

[gallenvara]

@fhueske ,thanks a lot for the review work! I'll modify the code and update the PR according to your advice.

[fhueske]

Thanks @gallenvara.
Flink uses Jackson to handle JSON data. You can use it to parse the JSON String.

Looking forward to your update :-)

[gallenvara]

@fhueske , codes has been finished. I have drop previous method of plan-generator and rewrite a new parser named PlanJsonParser to parse the existing JSON plan. Could you help with review work? :)

[rmetzger]

can you use the ${jackson.version} variable here?

[fhueske]

Hi @gallenvara, the PR looks much better. I added a few comment inline.

Thanks, Fabian

[gallenvara]

@fhueske , @rmetzger , thanks for review work. I have modified related code as you adviced and submitted a new commit.

[ChengXiangLi]

It's better to use some meaningful variable name here.