Electronics Research Lab Design Postgres 1985 - the Information ...

all on the component objects.5.3.6. Access MethodsWe expect to implement both B-tree and OB-tree [STON83b] secondaryindexes. Moreover, our ADT facility supports an arbitrary collection of userdefined indexes. Each such index is, in reality, a pair of indexes one for magneticdisk records and one for archival records. The first index is of the formindex-relation (user-key-or-keys, pointer-to-tuple)and uses the same structure as current ZNGRES eecondary indexes. The secondindex will have pointers to archival tuples and will add "tmin" and "tmax" towhatever user keys are declared. With this structure, records satisfying thequalification:where relation.key = valuewill be interpreted to mean:where (relation["now"].key = value)and will require searching only the magnetic disk index. General queries of theform:where relation[T].key = valuewill require searching both the magnetic disk and the archival index. Both indexesneed only search for records with qualifying keys; moreover the archival index canfurther restrict the search using tmax and tmin.Any POSTQUEL replace command will insert a new data record with anappropriate BXID and tmin, and then insert a record into all key indexes whichare defined, and lastly change tmax on the record to be updated. A POSTQUELappend will only perform the first and third steps while a delete only perfurms thesecond step. Providing a pointer from the old tuple to the new tuple would allowPOSTGRES to insert records only into indexes for keys that are modified. Thisoptimization saves many disk writes at some expense in run-time complexity. Weplan to implement this optimization.The implementor of a new access method structure need only keep in mindthat the new data record must be forced from main memory before any indexrecords (or the index record will point to garbage) and that multiple index updates(e.g. page splits) must be forced in the correct order (i.e. from leaf to root). This iseasily accomplished with a single low level command to the buffer manager:order pa gel, page2Inopportune crashes may leave an access method which consists of a multi-leveltree with dangling index pages (i.e. pages that are not pointed two from anywhereelse in the tree). Such crashes may also leave the heap with uncommitted datarecords that cannot be reached from some indexes. Such dangling tuples will begarbage collected by the vacuum process because they will have EXID equal to notcommitted. Unfortunately if dangling data records are not recorded in any index,then a sweep of memory will be periodicaly required to find them. Dangling indexepages must be garbage collected by conventional techniques.