feat: add explain TableFunction for substrait

This adds a new table function, `explain_substrait`, as well as a new
execution kernel for the function and binding kernel. This reuses the
`FromSubstraitFunctionData` structure since it contains all the pieces
we would need.

In a rebase, we re-add `FromSubstraitFunctionData` and we allow
ourselves to rename some functions since others we were matching the
style of have been removed. Changes upstream changed TableFunctions to
return a TableRef instead of a QueryResult, but to return an explain
plan I believe we still need to return a QueryResult (unless we
construct and return an `ExplainRelation` and call `GetTableRef` on
that).

src/substrait_extension.cpp

@@ -22,8 +22,8 @@
 
 namespace duckdb {
 
-void do_nothing(ClientContext *) {
-}
+//! This is a no-op deleter for creating a shared pointer to a reference.
+void deleter_noop(ClientContext *) {}
 
 struct ToSubstraitFunctionData : public TableFunctionData {
 	ToSubstraitFunctionData() = default;
@@ -264,7 +264,7 @@ static unique_ptr<TableRef> SubstraitBind(ClientContext &context, TableFunctionB
 		throw BinderException("from_substrait cannot be called with a NULL parameter");
 	}
 	string serialized = input.inputs[0].GetValueUnsafe<string>();
-	shared_ptr<ClientContext> c_ptr(&context, do_nothing);
+	shared_ptr<ClientContext> c_ptr(&context, deleter_noop);
 	auto plan = SubstraitPlanToDuckDBRel(c_ptr, serialized, is_json);
 	return plan->GetTableRef();
 }
@@ -275,6 +275,45 @@ static unique_ptr<TableRef> FromSubstraitBind(ClientContext &context, TableFunct
 
 static unique_ptr<TableRef> FromSubstraitBindJSON(ClientContext &context, TableFunctionBindInput &input) {
 	return SubstraitBind(context, input, true);
+
+//! Container for TableFnExplainSubstrait to get data from BindFnExplainSubstrait
+struct FromSubstraitFunctionData : public TableFunctionData {
+	FromSubstraitFunctionData() = default;
+	shared_ptr<Relation> plan;
+	unique_ptr<QueryResult> res;
+	unique_ptr<Connection> conn;
+};
+
+static unique_ptr<TableRef> BindFnExplainSubstrait(ClientContext &context, TableFunctionBindInput &input
+	                                                 vector<LogicalType> &return_types, vector<string> &names) {
+	if (input.inputs[0].IsNull()) {
+		throw BinderException("explain_substrait cannot be called with a NULL parameter");
+	}
+
+	// Prep args to `SubstraitPlanToDuckDBRel`
+	constexpr bool is_json = false;
+	string serialized = input.inputs[0].GetValueUnsafe<string>();
+	shared_ptr<ClientContext> c_ptr(&context, deleter_noop);
+
+	auto result  = make_uniq<FromSubstraitFunctionData>();
+	result->conn = make_uniq<Connection>(*context.db);
+	result->plan = SubstraitPlanToDuckDBRel(c_ptr, serialized, is_json);
+
+	// return schema is a single string attribute (column)
+	return_types.emplace_back(LogicalType::VARCHAR);
+	names.emplace_back("Explain Plan");
+
+	return std::move(result);
+}
+
+static void TableFnExplainSubstrait(ClientContext &context, TableFunctionInput &data_p, DataChunk &output) {
+	auto &data = data_p.bind_data->CastNoConst<FromSubstraitFunctionData>();
+	if (!data.res) { data.res = data.plan->Explain(); }
+
+	auto result_chunk = data.res->Fetch();
+	if (!result_chunk) { return; }
+
+	output.Move(*result_chunk);
 }
 
 void InitializeGetSubstrait(const Connection &con) {
@@ -299,22 +338,39 @@ void InitializeGetSubstraitJSON(const Connection &con) {
 	catalog.CreateTableFunction(*con.context, get_substrait_json_info);
 }
 
+//! Define and register a TableFunction ("from_substrait") that returns a TableRef
 void InitializeFromSubstrait(const Connection &con) {
-	auto &catalog = Catalog::GetSystemCatalog(*con.context);
-
-	// create the from_substrait table function that allows us to get a query
-	// result from a substrait plan
-	TableFunction from_sub_func("from_substrait", {LogicalType::BLOB}, nullptr, nullptr);
+	// `FromSubstraitBind` translates a substrait plan and returns a `TableRef`
+	// to return a `TableRef` we use `bind_replace` instead of `bind`
+	TableFunction from_sub_func("from_substrait", {LogicalType::BLOB}, nullptr);
 	from_sub_func.bind_replace = FromSubstraitBind;
+
+	// register the TableFunction in the system catalog
+	auto &catalog = Catalog::GetSystemCatalog(*con.context);
 	CreateTableFunctionInfo from_sub_info(from_sub_func);
 	catalog.CreateTableFunction(*con.context, from_sub_info);
 }
 
+//! Define and register a TableFunction ("explain_substrait") that returns a QueryResult
+void InitializeExplainSubstrait(const Connection &con) {
+	TableFunction explain_sub_func(
+	   "explain_substrait"
+	  ,{LogicalType::BLOB}
+	  ,/*function=*/TableFnExplainSubstrait // Translates the plan then converts to a string
+	  ,/*bind=*/BindFnExplainSubstrait      // Sets return schema to a single string
+	);
+
+	// register the TableFunction in the system catalog
+	auto &catalog = Catalog::GetSystemCatalog(*con.context);
+	CreateTableFunctionInfo explain_sub_info(explain_sub_func);
+	catalog.CreateTableFunction(*con.context, explain_sub_info);
+}
+
 void InitializeFromSubstraitJSON(const Connection &con) {
 	auto &catalog = Catalog::GetSystemCatalog(*con.context);
 	// create the from_substrait table function that allows us to get a query
 	// result from a substrait plan
-	TableFunction from_sub_func_json("from_substrait_json", {LogicalType::VARCHAR}, nullptr, nullptr);
+	TableFunction from_sub_func_json("from_substrait_json", {LogicalType::VARCHAR}, nullptr);
 	from_sub_func_json.bind_replace = FromSubstraitBindJSON;
 	CreateTableFunctionInfo from_sub_info_json(from_sub_func_json);
 	catalog.CreateTableFunction(*con.context, from_sub_info_json);
@@ -329,6 +385,7 @@ void SubstraitExtension::Load(DuckDB &db) {
 
 	InitializeFromSubstrait(con);
 	InitializeFromSubstraitJSON(con);
+	InitializeExplainSubstrait(con);
 
 	con.Commit();
 }