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

etrieve* into SUBORDWATES(E .name, E .mgr)from E in EMP, S in SUBORDINATESwhere E.name ="JoneswThis command continues to execute the Retrieve-into command until there are nochanges made to the SUBORDINATES relation.The "*' modifier can be appended to any of the POSTQUEL data manipulationcommands: Append, Delete, Execute, Replace, Retrieve, and Retzieve-into.Complex iterations, like the A-* heuristic search algorithm, can be specified usingsequences of these iteration queries [STON85b].Alerters and triggers are specified by adding the keyword "always" to a query.For example, an alerter is specified by a Retrieve command such asretrieve always (EMP.all)where EMP.name = "Bill"This command returns data to the application program that issued it wheneverBill's employee record is changed.' A trigger is an update query (i.e., Append,Replace, or Delete command) with an "always" keyword. For example, thecommanddelete always DEPTwhere count(EMP.name by DEPT.dnamewhere EMP.dept = DEPT.dname) = 0defines a trigger that will delete DEPT records for departments with no employees.Iteration queries differ from alerters and triggers in that iteration queries rununtil they cease to have an effect while alerters and triggers run indefinitely. Anefficient mechanism to awaken "always" commands is described in the systemarchitecture section."Always" commands support a forward-chaining control structure in which anupdate wakes up a collection of alerters and triggers that can wake up othercommands. This process terminates when no new commands are awakened.POSTGRES also provides support for a backward-chaining control structure.The conventional approach to supporting inference is to extend the viewmechanism (or something equivalent) with additional capabilities (e.g. IULLM85,WONG84, JARK851). The canonical example is the definition of the ANCESTORrelation based on a stored relation PARENT:PARENT (paren t-of, offspring)Ancestor can then be defined by the following commands:' Strictly speaking the data is returned to the program through a portal which isdefined in section 4.