| OCR Text |
Show 7 2.4.3 Obj ect Manip ulation Ce nera lly two modes of access to objects a re provided , 1.e ., access to a single object based on some object icl ent iAer or na rne, and access to a set of objects based on decla rat ive qu e ri es. This ra.ises a number of da tabase ma nagement issues, like index ma nagement, access p ath select ion, query opt imizat ion , and others. 2.4.4 C oncurrency Control & Recovery The t ra nsactions in a CAD system a re different from those in t radi t ional database ap pl icatio ns in Lbe sense t hat t hey regula rly extend over long periods of time and may involve la rge a mounts of data. This means t hat t be locki ng mech an isms slwuld he flexible so t hat t ransact ions do not have to wait for ea ch otber to finish and concuJTent t r a nsacti o ns s llo lt! cl be a ble to communicate t heir results to each other in t he middle of their execut ion . For recovery purposes, some syst ems use log mecha nisms whereas othe rs use shadow pages. Log mecha ni sms require before and d.flcr data images to be saved . T his may he inefficient for mul t imed ia data, so some :-:ystcms l iSP a shadow page mecha ni sm for such data. 2.4.5 Navig ational Support l11 g ra pl1-be:tsed stru ctures, a u obj(~c t can rdcr Lu a ny object residi ng on a secondary storage medium. T his means th at point er traversal co ul d be ext remely s lovv . To add ress t his, some sysL(-'IllS c luster objects whi ch refer to each ot her on t llf' seconda ry store, ami use p refetcl1ing and b11fleri ng . 2.4.6 Schem a Evolut ion Si1 1cP info rmat ion in a p e rsistent obj ect system may exist for many years, the n e(~ d to support evolu t ion and he te rogene ity is more acute t han it is in a t radi t iona l prog r<unming system [9]. T he prohle Jll i:-; to a llow ob jects to cb a.nge t l1 eir st rucLu rl' |
