Show optimized physical and logical plans in EXPLAIN

[alamb]

Which issue does this PR close?

Closes #221

Rationale for this change

"The EXPLAIN command is useful to show the plan of the query engine. However, the current output shows the un-optimized logical plan, which is not very useful as it maps almost 1:1 to the query."

Most users issue an EXPLAIN command to see what the query engine is actually going to run.

What changes are included in this PR?

Change EXPLAIN to show the optimized logical plan (after all logical optimizer passes) and optimized physical plan (after all physical optimizer passes).

Working with sql.rs reminded me of my goal for cleaning up sql tests, and so I filed #743

Are there any user-facing changes?

Explain plan output is different

For example, before this PR

> EXPLAIN SELECT c2 from foo;
+--------------+----------------------------------+
| plan_type    | plan                             |
+--------------+----------------------------------+
| logical_plan | Projection: #foo.c2              |
|              |   TableScan: foo projection=None |
+--------------+----------------------------------+
1 row in set. Query took 0.002 seconds.

and after this PR (note the inclusion of the projection pushdown and physical plan)

> EXPLAIN SELECT c2 from foo;
+---------------+--------------------------------------------------------------------------+
| plan_type     | plan                                                                     |
+---------------+--------------------------------------------------------------------------+
| logical_plan  | Projection: #foo.c2                                                      |
|               |   TableScan: foo projection=Some([1])                                    |
| physical_plan | ProjectionExec: expr=[c2@0 as c2]                                        |
|               |   RepartitionExec: partitioning=RoundRobinBatch(16)                      |
|               |     CsvExec: source=Path(/tmp/foo.csv: [/tmp/foo.csv]), has_header=false |
+---------------+--------------------------------------------------------------------------+

[alamb]

previously the physical planner could handle an LogicalPlan::Explain anywhere in the tree. However, I can't think if cases where this can actually occur and it made adding the final physical plan challenging.

So I have changed the code to special case handling Explain as the root.

[alamb]

This is basically a copy/paste from test/mod.rs -- I have plans to fix this in #743

[alamb]

FYI @Dandandan

[jimexist]

related #752 and #751 cc @alamb

[alamb]

FYI @Dandandan since you requested the original feature

[Dandandan]

Thanks, looks great! @alamb