▁ ▁is ▁either ▁the ▁distance ▁between ▁ ▁and ▁, ▁or ▁the ▁core ▁distance ▁of ▁, ▁wh iche ver ▁is ▁bigger : ▁ ▁If ▁ ▁and ▁ ▁are ▁nearest ▁neighb ors , ▁this ▁is ▁the ▁ ▁we ▁need ▁to ▁assume ▁to ▁have ▁ ▁and ▁ ▁belong ▁to ▁the ▁same ▁cluster . ▁ ▁Both ▁core - distance ▁and ▁reach ability - distance ▁are ▁undefined ▁if ▁no ▁sufficiently ▁dense ▁cluster ▁( w . r . t . ▁) ▁is ▁available . ▁Given ▁a ▁sufficiently ▁large ▁, ▁this ▁never ▁happens , ▁but ▁then ▁every ▁- ne igh bor hood ▁query ▁returns ▁the ▁entire ▁database , ▁resulting ▁in ▁ ▁runtime . ▁Hence , ▁the ▁ ▁parameter ▁is ▁required ▁to ▁cut ▁off ▁the ▁density ▁of ▁clusters ▁that ▁are ▁no ▁longer ▁interesting , ▁and ▁to ▁speed ▁up ▁the ▁algorithm . ▁ ▁The ▁parameter ▁ ▁is , ▁strictly ▁speaking , ▁not ▁necessary . ▁It ▁can ▁simply ▁be ▁set ▁to ▁the ▁maximum ▁possible ▁value . ▁When ▁a ▁spatial ▁index ▁is ▁available , ▁however , ▁it ▁does ▁play ▁a ▁practical ▁role ▁with ▁regards ▁to ▁complexity . ▁OP T IC S ▁abstract s ▁from ▁DB SC AN ▁by ▁removing ▁this ▁parameter , ▁at ▁least ▁to ▁the ▁extent ▁of ▁only ▁having ▁to ▁give ▁the ▁maximum ▁value . ▁ ▁Pse udo code ▁The ▁basic ▁approach ▁of ▁OP T IC S ▁is ▁similar ▁to ▁DB SC AN , ▁but ▁instead ▁of ▁maintain ing ▁a ▁set ▁of ▁known , ▁but ▁so ▁far ▁un process ed ▁cluster ▁members , ▁a ▁priority ▁queue ▁( e . g . ▁using ▁an