| OCR Text |
Show 51 t he end of t he sess ion. For such type of applications, bufFer martagement is not an important issue. 5.3.4 OlD support '1'\w () IIJ rr·prf'scnl at ion varies f"10m one !l OS to anoth er. ObServr-:r u:;cs 64 bit logintl O!Ds which arc> called externa.l UJDs. Tbesr--: extern al U!Ds a re mapped to internal UIDs. T'be in ternal UIDs are used to determine an obj ect 's location. As a resul t of using logical OIDS, ObServer can migrate objects from one site to a nother or rep li catc> t lwm on mul t iple sites. SSM usPs a struct ured OID which contains t he ohjecL's phys ical loc<lL ion . Postgres bas logical OfDs as a system-deGned att ri bute i11 t'very re lation. \·'rom Lit<' perrmmauce poin t of view, logical 0 IDs a re costly because they must be nmpped to t he act ual physical address of the obj ect a nd , in order to access the object, morf' t ba.n one di sk access may be needed. One in teresting and notable diff'erence between these POSs is t he relationship 1)('11\'('('JJ O!Us c\ Jid ol>.jccl s. In OIJS(' J'\'('J, 1.\i(' server, Oil IT <J II<'SL r i'Oi ll t lw clil'n t, pro\· id<·s a l l tllilher of OIDs which t lw cli ent can <Lltacb to any object a ncl sell([ bacl< l(J ti ll' ;:;c1·vcr for sto rage. In ESM, the client sends the object to the server which a ltac!Jes an OlD to t his object and retu rns the OlD to t lH:: cl ient . l_n Postgres, the cl ient sends the object to t he server and t he server assigns an 0 !D to t he object <~JJ d sto res iL. There is no wa r Lo ge1 t he 0 10 wit hout knowing tbe object. l11 Lil<' pres<'nt irnp !e JIH'D td.tioJt . the support prov icl ed by ObServer and 8SI\Il prov<·d to he adequate. One of Lbe rnain mot ivat ions beb.ind us.ing Postgres as a POS \\'as t hat it prov ides OlD support , but it tu r ned out t hat it cou lclnot be used lwcausc of the above limitat ion . Our imp lementat ion tJscs "pointer swizz ling" ancl t his rnea.ns that the r un-t ime system has to kuow tbc OlD of every obj ect it creates hecat1se t his object can be poten tiall ,v referen ced by other objects. Since th is is |
