Implement DESCRIBE SELECT to show schema rather than EXPLAIN plan

[djanderson]

Which issue does this PR close?

• Closes Make DESCRIBE SELECT 1; work #18234.

Rationale for this change

https://discord.com/channels/885562378132000778/1430237388474552380/1430618776751313018

What changes are included in this PR?

cargo run -q --bin datafusion-cli
DataFusion CLI v50.3.0
> DESCRIBE SELECT 1;
+-------------+-----------+-------------+
| column_name | data_type | is_nullable |
+-------------+-----------+-------------+
| Int64(1)    | Int64     | NO          |
+-------------+-----------+-------------+
1 row(s) fetched.
Elapsed 0.022 seconds.

Are these changes tested?

No, looking for feedback on approach first... happy to add a test.

Yes.

Are there any user-facing changes?

Yes, it changes to behavior of DESCRIBE SELECT from explaining the physical plan (EXPLAIN) to describing the schema of the query (like DESCRIBE table).

[adriangb]

Amazing! Could you just add some tests for both the working case and the error case? Maybe also with something slightly more interesting than SELECT 1;

[djanderson]

Amazing! Could you just add some tests for both the working case and the error case? Maybe also with something slightly more interesting than SELECT 1;

For sure. Is the NotImplemented error the route we want to take if the statement isn't a Query? If so, I'll add a few tests next.

[adriangb]

Amazing! Could you just add some tests for both the working case and the error case? Maybe also with something slightly more interesting than SELECT 1;

For sure. Is the NotImplemented error the route we want to take if the statement isn't a Query? If so, I'll add a few tests next.

Yes I just don't know when that would be hit, it would be good to have an example of when that code path gets hit for future reference.

[alamb]

Thank you @djanderson and @adriangb

Since this is a behavior change I think, we probably need to

1. Add a note in the upgrade guide
2. Add some test coverage (.slt style)

[adriangb]

Thank you for adding tests! I think the only thing missing is an entry in https://github.com/apache/datafusion/blob/main/docs/source/library-user-guide/upgrading.md

[djanderson]

@adriangb , done! But, as I was double checking currently documented behavior, I came upon https://datafusion.apache.org/user-guide/sql/ddl.html#describe, and I wasn't quite sure if a quick example about DESCRIBE query would fit appropriately within the DDL section?

[comphead]

I would suggest to use a tree schema representation which was added recently #17459

because it supports nested types, outputting plain table for nested types like in this PR might be confusing

[alamb]

This is great -- thank you @djanderson , @comphead and @adriangb

[djanderson]

I would suggest to use a tree schema representation which was added recently #17459

because it supports nested types, outputting plain table for nested types like in this PR might be confusing

@comphead, If you have an idea of how you'd like this approached, feel free to drop some notes in an issue and assign me! I looked into it a bit last night and couldn't figure out how to trivially make that change. Would it require a new DescribeQuery logical plan type so that queries and tables can have different describe output formats downstream?