add example

Author: paleolimbot

datafusion/sql/src/expr/mod.rs

@@ -1031,12 +1031,7 @@ impl<S: ContextProvider> SqlToRel<'_, S> {
             _ => expr,
         };
 
-        // Currently drops metadata attached to the type
-        // https://github.com/apache/datafusion/issues/18060
-        Ok(Expr::Cast(Cast::new(
-            Box::new(expr),
-            dt.data_type().clone(),
-        )))
+        Ok(Expr::Cast(Cast::new_from_field(Box::new(expr), dt)))
     }
 
     /// Extracts the root expression and access chain from a compound expression.

docs/source/library-user-guide/extending-sql.md

@@ -202,6 +202,51 @@ async fn main() -> Result<()> {
 }
 ```
 
+#### Example: Supporting the UUID Type
+
+```rust
+# use std::sync::Arc;
+# use arrow::datatypes::{FieldRef, TimeUnit};
+# use datafusion::error::Result;
+# use datafusion::prelude::*;
+# use datafusion::execution::SessionStateBuilder;
+use datafusion_expr::planner::TypePlanner;
+# use sqlparser::ast;
+
+#[derive(Debug)]
+struct MyTypePlanner;
+
+impl TypePlanner for MyTypePlanner {
+    fn plan_type_field(&self, sql_type: &ast::DataType) -> Result<Option<FieldRef>> {
+        match sql_type {
+            sqlparser::ast::DataType::Uuid => Ok(Some(Arc::new(
+                Field::new("", DataType::FixedSizeBinary(16), true).with_metadata(
+                    [("ARROW:extension:name".to_string(), "arrow.uuid".to_string())]
+                        .into(),
+                ),
+            ))),
+            _ => Ok(self
+                .plan_type(sql_type)?
+                .map(|data_type| data_type.into_nullable_field_ref())),
+        }
+    }
+}
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    let state = SessionStateBuilder::new()
+        .with_default_features()
+        .with_type_planner(Arc::new(MyTypePlanner))
+        .build();
+
+    let ctx = SessionContext::new_with_state(state);
+
+    // Now UUID type is recognized
+    ctx.sql("CREATE TABLE idx (uuid UUID)").await?;
+    Ok(())
+}
+```
+
 For more details, see the [TypePlanner API documentation].
 
 ### RelationPlanner: Custom FROM Clause Elements