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Show 21 2.5.11 Postgres J>ostg res [21] was clevelopecl at the Uni versity of California, Berkeley. As the S \\(T(-'SSO! ur t lw IJi g res re lat ional database system, Postgres extends it by adding clas:·ws, inheritance, types and f11nc.tions. It is progrcunming language neutral, 1.e., it cau lw c:allecl Crun1 many different languages. In Pustgres , eacb record gets a unique ident ifi er (OlD) which is a 32 bit quantity il lld is not r eusf~cl. \;\!h en a relation is cr<~atecl, a file is allocated to hold the records of" L!Jal relatiou. l{eco rcl s iu a. relation cau btc approximately clustered on tbe value of a 11ser designated field. Each disk record stores only fields which are non-null. In l)os1gres, daL<l is whah·vcr the user says it is. The Postgres backencl itse lf does not illlpus(' <tny arbit rary definition on data other than that it is a range of memory of a k11own size. Postgres n1aintains two different physical collections of records, one f"or tlw CIIJT<c!lL da.ta and one for bisturical data. One of the special f"c<tturcs of Postgres is tbat the transaction man agement does no1 11se write a head logging. No data is ow~ rw ritt c n, rather all updates are turned inLu insert ions. lt l1 as a \Jackground process called the vacmcm which compacts the <L( Liv<-> sto rt· by moving the older records to a separate archival store. It assumes llaicl ware support in the form of stable memory, protected by battery backup. This rrwans LhaL rr~ covery is instarJt a JWOI I"·· 1wc<lt1S<-' th ere is no code to run at recovery t. i11w. AcldiLiumtlly, ti l (-~ I!Ser cau keel) th e ent ire past bistury. Couctn-rency coutrol i\11 011) <t ssur'irtt cc! witb a rf·ccml can be us<-·cl a.c; a clata item iu another r<-~ conl. L si11g i1Hiiu·s on OID~; . it is possible to navigate fron1 one record to the next by runn ing one query per navigatiou . A class cat.! lJ·e ,·1. v<~r·s-:J<)t-.1 <·.J·f·,".l.tl<-Jti·l<-'.1. <'le>c·c· lr1 t·l11·,. <·. - " '"'"'" . , ,, _.·(-\ s·",_' , J.t ,·s· c·;· tot·e<. ·l a"s· ··c t uJJ ·f·f· ere·1· 1· t·, ·le ·t I rl'la t iv<' to its parent class. Destmyiug a class which is a parent of a version class is disallovved. |
