KEYUSE

typedef struct keyuse_t {
  TABLE *table;
  Item    *val;                /**< or value if no field */
  table_map used_tables;
  uint    key, keypart, optimize;
  key_part_map keypart_map;
  ha_rows      ref_table_rows;
  /**
    If true, the comparison this value was created from will not be
    satisfied if val has NULL 'value'.
  */
  bool null_rejecting;
  /*
    !NULL - This KEYUSE was created from an equality that was wrapped into
            an Item_func_trig_cond. This means the equality (and validity of 
            this KEYUSE element) can be turned on and off. The on/off state 
            is indicted by the pointed value:
              *cond_guard == TRUE <=> equality condition is on
              *cond_guard == FALSE <=> equality condition is off

    NULL  - Otherwise (the source equality can't be turned off)
  */
  bool *cond_guard;
} KEYUSE;