Error while running COUNT DISTINCT (timestamp): 'Unexpected DataType for list

[alamb]

Describe the bug
A query panic'd on me

To Reproduce
Make a local file

echo "2018-11-13T17:11:10.011" > /tmp/foo.csv
echo "2018-12-13T12:12:10.011" >> /tmp/foo.csv
echo "2018-12-13T12:12:10.011" >> /tmp/foo.csv
echo "2018-12-13T12:13:10.011" >> /tmp/foo.csv

Run a query with datafusion-cli

cargo run -p datafusion-cli
    Finished dev [unoptimized + debuginfo] target(s) in 0.12s
     Running `target/debug/datafusion-cli`
> CREATE EXTERNAL TABLE foo(ts TIMESTAMP)
STORED AS CSV
LOCATION '/tmp/foo.csv';

0 rows in set. Query took 0 seconds.
> SELECT COUNT(DISTINCT ts) from foo;

thread 'tokio-runtime-worker' panicked at 'Unexpected DataType for list', datafusion/src/scalar.rs:366:22
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'tokio-runtime-worker' panicked at 'Unexpected DataType for list', datafusion/src/scalar.rs:366:22
thread 'tokio-runtime-worker' panicked at 'Unexpected DataType for list', datafusion/src/scalar.rs:366:22
thread 'tokio-runtime-worker' panicked at 'Unexpected DataType for list', datafusion/src/scalar.rs:366:22
thread 'tokio-runtime-worker' panicked at 'Unexpected DataType for list', datafusion/src/scalar.rs:366:22
thread 'tokio-runtime-worker' panicked at 'Unexpected DataType for list', datafusion/src/scalar.rs:366:22
thread 'tokio-runtime-worker' panicked at 'thread 'Unexpected DataType for listthread '', tokio-runtime-workerdatafusion/src/scalar.rstokio-runtime-worker' panicked at ':' panicked at 'Unexpected DataType for list366Unexpected DataType for list', :', thread 'datafusion/src/scalar.rsdatafusion/src/scalar.rstokio-runtime-worker22:thread '' panicked at ':
366366Unexpected DataType for list:tokio-runtime-worker:', 22thread '22datafusion/src/scalar.rs' panicked at 'tokio-runtime-worker
Unexpected DataType for list' panicked at '
', :Unexpected DataType for listdatafusion/src/scalar.rs366thread '::tokio-runtime-worker366', ' panicked at ':22datafusion/src/scalar.rsUnexpected DataType for list
22', :
datafusion/src/scalar.rsthread '366:tokio-runtime-worker:366' panicked at '22:Unexpected DataType for list
22', 
datafusion/src/scalar.rs:366:22
thread 'tokio-runtime-worker' panicked at 'Unexpected DataType for list', ArrowError(ExternalError(Canceled))
datafusion/src/scalar.rs:366:22
> thread 'tokio-runtime-worker' panicked at 'Unexpected DataType for list', datafusion/src/scalar.rs:366:22

Expected behavior
I expect a result to be produced with 3 distinct timestamp values

[charlibot]

Hi @alamb, I had a go at resolving this in #319.

[alamb]

Thanks @charlibot !