 
   FOREIGN KEY (node_id) REFERENCES gene_tree_node(node_id),
 
+	-- NO PK because unicity is not enforced
+
   KEY node_id_tag (node_id, tag),
   KEY tag (tag)
 

  FOREIGN KEY (dnafrag_id) REFERENCES dnafrag(dnafrag_id),
  FOREIGN KEY (method_link_species_set_id) REFERENCES method_link_species_set(method_link_species_set_id),

  KEY constrained_element_id_idx (constrained_element_id),
  KEY mlssid_idx (method_link_species_set_id),
  KEY mlssid_dfId_dfStart_dfEnd_idx (method_link_species_set_id,dnafrag_id,dnafrag_start,dnafrag_end),

  value                  mediumtext NOT NULL,

  FOREIGN KEY (root_id) REFERENCES gene_tree_root(root_id),

	-- NO PK because unicity is not enforced

  KEY root_id_tag (root_id, tag),
  KEY (root_id),
