[Python] Add support for `df.show()` on `CREATE EXTERNAL TABLE` statements

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

>>> df = ctx.sql("CREATE EXTERNAL TABLE orders STORED AS PARQUET LOCATION '/mnt/bigdata/tpch/sf1-parquet/orders'")
>>> df.show()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception: Arrow error: External error: Execution error: Job hZh0A3x failed: Error planning job hZh0A3x: DataFusionError(Internal("Unsupported logical plan: CreateExternalTable"))

Describe the solution you'd like
Make this work

Describe alternatives you've considered
None

Additional context
None

[andygrove]

It actually does work but we get an error if we try and show the results of the DataFrame. This is not good UX so we will need to fix.

>>> import ballista
>>> ctx = ballista.BallistaContext("localhost", 50050)
>>> ctx.sql("CREATE EXTERNAL TABLE orders STORED AS PARQUET LOCATION '/mnt/bigdata/tpch/sf1-parquet/orders'")
<ballista.DataFrame object at 0x7f501f088150>
>>> df = ctx.sql("SELECT count(*) FROM orders")
>>> df.show()
+-----------------+
| COUNT(UInt8(1)) |
+-----------------+
| 14999999        |
+-----------------+