Add format option for PostgreSQL explain

[pawurb]

This PR adds JSON and YAML format outputs to explain for PostgreSQL databases:

Repo.explain(:all, Area, format: :json) |> Jason.decode!()

[
  %{
    "Plan" => %{
      "Alias" => "a0",
      "Node Type" => "Seq Scan",
      "Parallel Aware" => false,
      "Plan Rows" => 70,
      "Plan Width" => 1064,
      "Relation Name" => "areas",
      "Startup Cost" => 0.0,
      "Total Cost" => 10.7
    }
  }
]

Repo.explain(:all, Area, format: :yaml) |> IO.puts

- Plan:
    Node Type: "Seq Scan"
    Parallel Aware: false
    Relation Name: "areas"
    Alias: "a0"
    Startup Cost: 0.00
    Total Cost: 10.70
    Plan Rows: 70
    Plan Width: 1064

Especially the json format is useful because it can be used with query visualizer tool for some advanced debugging.

[josevalim]

Thanks @pawurb, this looks great!

I only have one question: the YAML format is still returned as text while JSON returns a map. This feels a bit inconsistent to me. I wonder if instead we should introduce something like format: :map?

/cc @leandrocp

[pawurb]

@josevalim I'm afraid that parsing YAML would require adding another dependency. How about we stick to the text format for all the outputs and leave the parsing for the client?

[josevalim]

@josevalim I'm afraid that parsing YAML would require adding another dependency. How about we stick to the text format for all the outputs and leave the parsing for the client?

That's another route. We keep it all as text and format: :map can be a convenience added later on.

[leandrocp]

How about we stick to the text format for all the outputs and leave the parsing for the client?

Agreed, IMO that's the best option 👍🏻

[josevalim]

Yes, looks good to me, sorry for the confusion! I think we should not provide JSON then until we figure out the Jaosn dependency, so I will remove it post-merge. Thank you!

[josevalim]

💚 💙 💜 💛 ❤️