Implement DESCRIBE <table>

[LiuYuHui]

Which issue does this PR close?

Closes #2606.

Rationale for this change

What changes are included in this PR?

When the user executes the DESCRIBE <table>, the output will be the following.

❯ create table foo as select * from (values (1), (3), (2), (10), (8));
+---------+
| column1 |
+---------+
| 1       |
| 3       |
| 2       |
| 10      |
| 8       |
+---------+
5 rows in set. Query took 0.013 seconds.
❯ describe foo;
+-------------+-----------+-------------+
| column_name | data_type | is_nullable |
+-------------+-----------+-------------+
| column1     | Int64     | YES         |
+-------------+-----------+-------------+
1 row in set. Query took 0.016 seconds.

Are there any user-facing changes?

Does this PR break compatibility with Ballista?

[liukun4515]

LGTM

[alamb]

Thank you for the contribution @LiuYuHui ! This is a great start.

I tried it out locally and it doesn't seem to work quite right yet:

(arrow_dev) alamb@MacBook-Pro-6:~/Software/arrow-datafusion/datafusion-cli$ cargo run
...
DataFusion CLI v8.0.0
❯ CREATE OR REPLACE TABLE y AS VALUES (1,2),(3,4);
+---------+---------+
| column1 | column2 |
+---------+---------+
| 1       | 2       |
| 3       | 4       |
+---------+---------+
2 rows in set. Query took 0.062 seconds.
❯ describe table;
0 rows in set. Query took 0.032 seconds.
❯ describe table y;  🤔 Invalid statement: sql parser error: Expected end of statement, found: y

Also, can you please add a test (perhaps you can use he example above).

It might be nice to add a test here:
https://github.com/LiuYuHui/arrow-datafusion/blob/describe_table/datafusion/core/tests/sql/information_schema.rs#L223

which you can run with a command like cargo test --test sql_integration

[LiuYuHui]

Hi @alamb, thanks for your review, currently this PR only implements describe table_name, the describe table table_name is not implemented, this is why your test case has an error, should I also implement the describe table table_name?

[alamb]

Hi @alamb, thanks for your review, currently this PR only implements describe table_name, the describe table table_name is not implemented, this is why your test case has an error, should I also implement the describe table table_name?

🤦 -- no sorry @LiuYuHui I was mistaken on the syntax of describe -- no need to implement describe table foo

I tried the correct syntax and it works great 👍

❯ describe y;
+-------------+-----------+-------------+
| column_name | data_type | is_nullable |
+-------------+-----------+-------------+
| column1     | Int64     | YES         |
| column2     | Int64     | YES         |
+-------------+-----------+-------------+
2 rows in set. Query took 0.006 seconds.

However, I do think this PR needs a test.

Also, would it be possible to generate an error if the table didn't exist?

For example, mysql does the following for a non existent table:

mysql> describe ff;
ERROR 1146 (42S02): Table 'foo.ff' doesn't exist

But this PR returns an empty row:

❯ describe table;
0 rows in set. Query took 0.020 seconds.

[LiuYuHui]

@alamb, I have fixed the nonexistent table case and added the test, please take a look.

[alamb]

This PR looks great now @LiuYuHui -- thank you very much

I tried it out locally too and 👍