 @table ontology_xref
 @desc This table associates ontology terms/accessions to Ensembl objects (primarily EFO/SO). NOTE: Currently not in use
 
-@column object_xref_id          Composite key. Foreign key references to the @link object_xref table.
-@column source_xref_id          Composite key. Foreign key references to the @link xref table.
-@column linkage_type            Composite key.
+@column object_xref_id          Foreign key references to the @link object_xref table.
+@column source_xref_id          Foreign key references to the @link xref table.
+@column linkage_type            Defines type of linkage
 
 @see object_xref
 

@table external_synonym
@desc Some xref objects can be referred to by more than one name. This table relates names to xref IDs.

@column xref_id           Primary key, internal identifier.
@column synonym           Synonym

@see @link xref

@table  experimental_design
@desc   Denormalised link table to allow many to many design_type associations

@column design_type_id			@link design_type table ID
@column table_name				Name of linked table
@column table_id				Table ID of linked record

@see @link design_type
*/
