Support synthetic keys in the (relations ...) CSV loading form

meta/src/meta/grammar.y

@@ -88,7 +88,7 @@
 %nonterm gnf_column_path Sequence[String]
 %nonterm gnf_columns Sequence[logic.GNFColumn]
 %nonterm named_column logic.NamedColumn
-%nonterm relation_keys Sequence[logic.NamedColumn]
+%nonterm relation_keys Tuple[Sequence[logic.NamedColumn], Boolean]
 %nonterm target_relation logic.TargetRelation
 %nonterm non_cdc_relations Sequence[logic.TargetRelation]
 %nonterm cdc_inserts Sequence[logic.TargetRelation]
@@ -1134,6 +1134,12 @@ named_column
 
 relation_keys
     : "(" "keys" named_column* ")"
+      construct: $$ = builtin.tuple($3, False)
+      deconstruct if not $$[1]:
+        $3: Sequence[logic.NamedColumn] = $$[0]
+    | "(" "keys" "synthetic" ")"
+      construct: $$ = builtin.tuple(list[logic.NamedColumn](), True)
+      deconstruct if $$[1]:
 
 target_relation
     : "(" "relation" relation_id named_column* ")"
@@ -1166,7 +1172,7 @@ target_relations
     : "(" "relations" relation_keys relation_body ")"
       construct: $$ = construct_relations($3, $4)
       deconstruct:
-        $3: Sequence[logic.NamedColumn] = $$.keys
+        $3: Tuple[Sequence[logic.NamedColumn], Boolean] = deconstruct_relation_keys($$)
         $4: logic.TargetRelations = $$
 
 csv_locator_paths
@@ -1558,13 +1564,19 @@ def construct_cdc_relations(
     )
 
 
+def deconstruct_relation_keys(
+    msg: logic.TargetRelations,
+) -> Tuple[Sequence[logic.NamedColumn], Boolean]:
+    return builtin.tuple(msg.keys, msg.synthetic_key)
+
+
 def construct_relations(
-    keys: Sequence[logic.NamedColumn],
+    keys: Tuple[Sequence[logic.NamedColumn], Boolean],
     body: logic.TargetRelations,
 ) -> logic.TargetRelations:
     if builtin.has_proto_field(body, "plain"):
-        return logic.TargetRelations(keys=keys, plain=body.plain)
-    return logic.TargetRelations(keys=keys, cdc=body.cdc)
+        return logic.TargetRelations(keys=keys[0], synthetic_key=keys[1], plain=body.plain)
+    return logic.TargetRelations(keys=keys[0], synthetic_key=keys[1], cdc=body.cdc)
 
 
 def construct_csv_data(

proto/relationalai/lqp/v1/logic.proto

@@ -315,11 +315,14 @@ message CDCTargets {
 // Generalized loading: shared key columns plus the target relations, loaded either as a
 // plain snapshot or as CDC insert/delete deltas. The two modes are mutually exclusive.
 message TargetRelations {
-  repeated NamedColumn keys = 1; // Shared key columns
+  repeated NamedColumn keys = 1; // Shared key columns; must be empty when synthetic_key is set
   oneof body {
     PlainTargets plain = 2;
     CDCTargets cdc = 3;
   }
+  // If true, the shared key is synthesized by the loader (e.g. a generated row key)
+  // instead of being drawn from CSV columns. Mutually exclusive with `keys`.
+  bool synthetic_key = 4;
 }
 
 message CSVData {