Implement DESCRIBE SELECT to show schema rather than EXPLAIN plan (#18238)

## Which issue does this PR close?

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- Closes #18234.

## Rationale for this change

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in the issue then this section is not needed.
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your changes and offer better suggestions for fixes.
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https://discord.com/channels/885562378132000778/1430237388474552380/1430618776751313018

## What changes are included in this PR?

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```
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.

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## 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).

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updated before approving the PR.
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Author: djanderson

datafusion/sql/src/statement.rs

@@ -242,6 +242,16 @@ impl<S: ContextProvider> SqlToRel<'_, S> {
                 table_name,
                 ..
             } => self.describe_table_to_plan(table_name),
+            Statement::Explain {
+                describe_alias: DescribeAlias::Describe | DescribeAlias::Desc, // only parse 'DESCRIBE statement' or 'DESC statement' and not 'EXPLAIN statement'
+                statement,
+                ..
+            } => match *statement {
+                Statement::Query(query) => self.describe_query_to_plan(*query),
+                _ => {
+                    not_impl_err!("Describing statements other than SELECT not supported")
+                }
+            },
             Statement::Explain {
                 verbose,
                 statement,
@@ -1399,6 +1409,19 @@ impl<S: ContextProvider> SqlToRel<'_, S> {
         }))
     }
 
+    fn describe_query_to_plan(&self, query: Query) -> Result<LogicalPlan> {
+        let plan = self.query_to_plan(query, &mut PlannerContext::new())?;
+
+        let schema = Arc::new(plan.schema().as_arrow().clone());
+
+        let output_schema = DFSchema::try_from(LogicalPlan::describe_schema()).unwrap();
+
+        Ok(LogicalPlan::DescribeTable(DescribeTable {
+            schema,
+            output_schema: Arc::new(output_schema),
+        }))
+    }
+
     fn copy_to_plan(&self, statement: CopyToStatement) -> Result<LogicalPlan> {
         // Determine if source is table or query and handle accordingly
         let copy_source = statement.source;

datafusion/sqllogictest/test_files/describe.slt

@@ -116,3 +116,29 @@ col1 Int32 YES
 # Test error cases
 statement error
 DESC nonexistent_table;
+
+##########
+# Describe statement
+##########
+
+# Test describing the schema of a simple statement
+query TTT
+DESCRIBE SELECT 1;
+----
+Int64(1) Int64 NO
+
+# Insert some data into the existing test table...
+statement ok
+INSERT INTO test_desc_table (id, name) VALUES (1, 'Alice'), (2, 'Bob'), (3, 'Charlie'), (4, 'Alice');
+
+# ... and describe the schema of a more complex query
+query TTT
+DESCRIBE SELECT name, COUNT(*) AS name_count FROM test_desc_table
+    GROUP BY name HAVING COUNT(*) > 1 ORDER BY name_count DESC;
+----
+name Utf8View YES
+name_count Int64 NO
+
+# Describing a statement that's not a query is not supported
+statement error Describing statements other than SELECT not supported
+DESCRIBE CREATE TABLE test_desc_table (id INT, name VARCHAR);

docs/source/library-user-guide/upgrading.md

@@ -116,6 +116,12 @@ Users may need to update their paths to account for these changes.
 
 See [issue #17713] for more details.
 
+### `DESCRIBE query` support
+
+`DESCRIBE query` was previously an alias for `EXPLAIN query`, which outputs the
+_execution plan_ of the query. With this release, `DESCRIBE query` now outputs
+the computed _schema_ of the query, consistent with the behavior of `DESCRIBE table_name`.
+
 ## DataFusion `50.0.0`
 
 ### ListingTable automatically detects Hive Partitioned tables