ARROW-12278: [Rust][DataFusion] Use Timestamp(Nanosecond, None) for SQL TIMESTAMP Type

[alamb]

Rationale

Running the query CREATE EXTERNAL TABLE .. (c TIMESTAMP) today in DataFusion will result in a data type pf "Date64" which means that anything more specific than the date will be ignored.

This leads to strange behavior such as

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

cargo run -p datafusion --bin datafusion-cli
    Finished dev [unoptimized + debuginfo] target(s) in 0.23s
     Running `target/debug/datafusion-cli`
> CREATE EXTERNAL TABLE t(a varchar, b TIMESTAMP)
STORED AS CSV
LOCATION '/tmp/foo.csv';

0 rows in set. Query took 0 seconds.
> select * from t;
+--------+------------+
| a      | b          |
+--------+------------+
| Andrew | 2018-11-13 |
| Jorge  | 2018-12-13 |
+--------+------------+

(note that the Time part is chopped off)

Changes

This PR changes the default mapping from SQL type TIMESTAMP

[alamb]

This is needed so that CREATE EXTERNAL TABLE is handled correctly

[alamb]

This is the actual code change in this PR

[github-actions]

https://issues.apache.org/jira/browse/ARROW-12278

[alamb]

FYI @Dandandan / @seddonm1 / @ovr

[jorgecarleitao]

LGTM. Nice catch.

Random though: I think that we would benefit from not writing a test on Context to test the SQL planner. It could be easier to identify which tests are testing what by writing unit tests more specialized to the component that they affect, as they usually declare the API behavior and are easier to find.

[jorgecarleitao]

a wild println has appeared :)

[alamb]

Oh no! I will remove

[Dandandan]

👍

[alamb]

Random though: I think that we would benefit from not writing a test on Context to test the SQL planner. It could be easier to identify which tests are testing what by writing unit tests more specialized to the component that they affect, as they usually declare the API behavior and are easier to find.

@jorgecarleitao I think the core of the problem is that the tests in context.rs are largely end to end tests of DataFusion rather than tests for the ExecutionContext itself. I think they ended up in context.rs because that is typically the main entry point to run queries.

What would you think about refactoring the non ExecutionContext specific tests to some other file (perhaps the tests/sql.rs end to end test?)