input: [Option name] index_structure [Option type] string [Example] INGRES_STRUCTURE_HASH [Description] 
output: The default structure for new secondary indexes. Valid values for index_structure are: INGRES_STRUCTURE_CBTREE INGRES_STRUCTURE_CISAM INGRES_STRUCTURE_CHASH INGRES_STRUCTURE_BTREE INGRES_STRUCTURE_HASH INGRES_STRUCTURE_ISAM