Method ha_start_keyread

sql/handler.h:3779–3784

  inline bool keyread_enabled() { return keyread < MAX_KEY; }
  inline int ha_start_keyread(uint idx)
  {
    DBUG_ASSERT(!keyread_enabled());
    keyread= idx;
    return extra_opt(HA_EXTRA_KEYREAD, idx);
  }
  inline int ha_end_keyread()
  {
    if (!keyread_enabled())                    /* Enable lazy usage */