Fix "ON CONFLICT ON CONSTRAINT" on plain PostgreSQL tables

In the planner hook, we erroneously blocked "ON CONFLICT ON
CONSTRAINT" statements on plain tables. This fixes the issue
so that this error only occurs on hypertables.

src/planner.c

@@ -368,13 +368,6 @@ modifytable_plan_walker(Plan **planptr, void *pctx)
 			ListCell   *lc_plan,
 					   *lc_rel;
 
-			if (ctx->parse->onConflict != NULL &&
-				ctx->parse->onConflict->constraint != InvalidOid)
-				ereport(ERROR,
-						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-						 errmsg("Hypertables do not support ON CONFLICT statements that reference constraints"),
-					 errhint("Use column names to infer indexes instead.")));
-
 			/*
 			 * To match up tuple-producing subplans with result relations, we
 			 * simultaneously loop over subplans and resultRelations, although
@@ -391,6 +384,13 @@ modifytable_plan_walker(Plan **planptr, void *pctx)
 					void	  **subplan_ptr = &lfirst(lc_plan);
 					Plan	   *subplan = *subplan_ptr;
 
+					if (ctx->parse->onConflict != NULL &&
+						ctx->parse->onConflict->constraint != InvalidOid)
+						ereport(ERROR,
+								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+								 errmsg("Hypertables do not support ON CONFLICT statements that reference constraints"),
+								 errhint("Use column names to infer indexes instead.")));
+
 					/*
 					 * We replace the plan with our custom chunk dispatch
 					 * plan.