Expose Constraints internals

[tv42]

Describe the bug

DataFusion v29.0.0 added commit 5f03146, which contains this change:

diff --git a/datafusion/expr/src/logical_plan/ddl.rs b/datafusion/expr/src/logical_plan/ddl.rs
index e005f1147..dc247da36 100644
--- a/datafusion/expr/src/logical_plan/ddl.rs
+++ b/datafusion/expr/src/logical_plan/ddl.rs
@@ -222,8 +217,8 @@ impl Hash for CreateExternalTable {
 pub struct CreateMemoryTable {
     /// The table name
     pub name: OwnedTableReference,
-    /// The ordered list of columns in the primary key, or an empty vector if none
-    pub primary_key: Vec<Column>,
+    /// The list of constraints in the schema, such as primary key, unique, etc.
+    pub constraints: Constraints,
     /// The logical plan
     pub input: Arc<LogicalPlan>,
     /// Option to not error if table already exists

So, previously, primary_key was just a Vec, and Column was public. Now it's Constraints, and Constraints has only private fields and no accessor methods: https://docs.rs/datafusion/latest/datafusion/common/struct.Constraints.html

Previously, I could take a logical plan and introspect it. Now I can't.

Can we make Constraint public, and either make the inner of Constraints public, or add an accessor method (maybe an iterator)?

The change should be pretty simple, but I'm trying to understand the thinking behind the current API.

Cc @mustafasrepo @metesynnada @ozankabak

To Reproduce

        let statement = session_state.sql_to_statement(sql, sql_dialect)?;
        let logical_plan = session_state.statement_to_plan(statement).await?;

        match logical_plan {
            datafusion::logical_expr::LogicalPlan::Ddl(
                datafusion::logical_expr::DdlStatement::CreateMemoryTable(create_table),
            ) => {
                for key in &create_table.primary_key {
                    println!("key {} ref {:?}", key.name, key.relation);
                }
            _ => (),
        }

Expected behavior

Be able to introspect the LogicalPlan.

Additional context

No response

[ozankabak]

Can we make Constraint public, and either make the inner of Constraints public, or add an accessor method (maybe an iterator)?

This sounds reasonable to me.

[alamb]

I think this would be a great first issue:

1. Add Constraints::iter() and Contraints::into_inner() --> Vec<Constraint>
2. Write some tests

[tv42]

I have a simple patch that implements Deref to &[Constraint] -- that seems like the right thing, and gives iteration for free. I need to read the contributing guide, but the code is trivial.

[alamb]

Thanks @tv42