Heuer, Andreas

Kröger, Joachim

Paul, Stefan

Universität Rostock, Lehrstuhl Datenbank- und Informationssysteme: Dbis Repository

On the Ordering of Rewrite RulesExtended AbstractJoachim Kroger  Stefan Paul and Andreas Heuer  University of Rostock Computer Science Department DB Research GroupD Rostock Germanyfjo heuerginformatikunirostockdehttpwwwdbinformatikunirostockdeResearchCROQUEenglhtmlco SAG Systemhaus GmbH Brandst	ucken  D Hamburg GermanystpsoftwareagdeAbstract The conceptual development of the rulebased component ofthe CROQUE query rewrite and optimization system led to the derivation of an ordering of the rules present in the rule base according to theiroptimization potential in order to increase the eciency of the logical term rewriting  This heuristic may indeed be used for any otherrulebased optimizer tooThe major contribution of our approach is a combination of three ideas limit the search space of query optimization by grouping and orderingrules for rulebased optimizers  use of oine preoptimizationordering instead of dynamic ordering during the optimization process taking into consideration more than one nbest alternatives forfurther evaluation  MotivationThe CROQUE project   is concerned with dierent aspects of optimizationand evaluation of object	oriented queries Starting point of all our considerationsare queries in ODMGs OQL  formalized in  that are rst representedinternally employing a hybrid approach of calculus and algebra  before beingtransformed using a rule	 and cost	based optimizerIn general equivalence rules may be applied in both directions Since inmost cases a heuristically preferred direction of application can easily be distinguished we only consider directed rules transformation rules in the frameworkof the CROQUE project Certain normalization steps are separately realizedrst splitting the rule set in smaller subsets Thus not all dened rules are partof the rewriting according to the described conceptAn exhaustive matching of all rules against every operator node does notmake sense since most of the rules may not match successfully against most of CROQUE Cost and Rulebased Optimization of objectoriented QUEries is a jointresearch project of the universities of Rostock and Konstanz funded by the GermanResearch Association DFG under contracts He  and Scho the considered nodes Eciency is increased by dividing the rule set up into ruleclasses using the root node of the left hand side of the rule for classication Inthis way the set of rules to be considered in a sub	query tree rewriting may bereduced to exactly one rule class by only one single function call The overheadneeded for pattern matching will hereby be cut down considerably Ordering Rewrite RulesRealizing the real need of a heuristic search space pruning we developed a heuristic that is the present focus of our interest per sub	query tree rewriting nomore than the n best rules shall be applied Therefore we have to order therules Additionally this strategy also succeeds in minimizing the number of pattern matcher calls The ordering of rules is not a new idea but in other approachesthe ordering has to be programmed in general or estimation of the rule orderingis done online A somewhat more detailed discussion of other approaches comprising the systems COKO	KOLA Volcano EXODUS Starburst GOM andGral may be found in the full paper The realization of the rewriting in CROQUE is as follows A given logicalexpression is rewritten by the rewrite engine according to a given strategy thusspanning a logical search space Therefore each call to the pattern matchersupplies a sub	query tree t and an ordered list r of rules The elements of rare matched against t in order User queries successfully applied rules andassociated rewriting results which altogether dene the systems query loadare passed to the adaptation component which may then possibly inuence therule orderingIn order to minimize the overhead we avoid an online ordering during thequery evaluation ie the rules are applied according to their order in the considered rule classes list An initial ordering of the rules is done only once in thebeginning Afterwards the ordering is modied during distinguished adaptationphases These steps may be done according to any policy the only request wedemand for is that the adaptation is done oine By means of the adaptationpossible mistakes or inaccuracies of the initial ordering shall be remedied andadditionally the ordering shall be adapted to cover the characteristics of theconsidered database eg the used schema present instances and the observedquery load  Initial Rule OrderingThe initial ordering of the rules is realized according to a very simple methodtwo rules are comparable ie may be ordered if there exist a smallest commonpattern that is contained in both rules Applying both rules to this pattern willlead to two expressions that may easily be ranked by a human query optimization expert eg the DBA Moreover the ordering is assumed to be transitiveIncomparable rules are ordered randomly in the initial ordering phase The isolated rule orderings obtained in this way are integrated for every rule class bydetermining their topological order   Rule Order AdaptationThe adaptation of the initial ordering is done by the help of selected statisticsFurthermore a combination of benchmarks and specic user queries is utilizedfor an oine rule assessment Statistics are mainly intended as a means to helpordering incomparable rules since a random ordering is not really satisfyingRules that may be used very often with a noticeable success are ranked higherthan rarely useable rules that only result in small cost reductions So	calledbenchmarks are used to correct the ranking done by hand in the initial ruleordering phase Overall Optimization ProcessOptimization in CROQUE is not restricted to algebraic rewriting The rewritingof expressions is done in a hybrid approach consisting of a calculus and an algebranotation  Rewriting calculus expressions is done in the same way as rewritingalgebraic expressions by means of rules and pattern matching Thus the resultsare transferable The use of heuristics therefore integrates well into the relatedwork of the CROQUE projectEvaluation of all our concepts will be done on the basis of our query optimizerprototype Most parts of its implementation are already complete so that theoptimizer is nearly fully operational There will be further work in improvingour heuristics Above all the automatic adaptation of the rule ordering willbe a topic of our ongoing investigations as well as investigations about furtheropportunities for adaptation on other optimizer layersReferences RGG Cattell editor The Object Database Standard ODMG Release 	MorganKaufmann San Mateo CA  D Gluche T Grust C Mainberger and MH Scholl Incremental Updates for Materialized Views with UserDened Functions In Proc of the Fifth Int Conferenceon Deductive and ObjectOriented Databases DOOD Montreux SwitzerlandDecember  T Grust J Kr	oger D Gluche A Heuer and MH Scholl Query Evaluation inCROQUE  Calculus and Algebra Coincide In Proc of the th British NationalConference on Databases BNCOD  London UK LNCS 	 Springer pages July  A Heuer and J Kr	oger Query Optimization in the CROQUE Project In Proc ofthe th Int Conference on Database and Expert Systems Applications DEXA Zurich Switzerland LNCS  Springer pages  September  J Kr	oger S Paul and A Heuer Query Optimization On the Ordering of RulesPreprint CS CS Dept University of Rostock Available at httpwwwdbinformatikunirostockdejoCShtml H Riedel and MH Scholl A Formalization of ODMG Queries In Proc of the thInt Conference on Database Semantics DS Leysin Switzerland October 

On the Ordering of Rewrite Rules (Extended Abstract)

http://eprints.dbis.informatik.uni-rostock.de/705/1/adbis.pdf

On the Ordering of Rewrite Rules (Extended Abstract)

Abstract

Similar works

Full text

Available Versions

Universität Rostock, Lehrstuhl Datenbank- und Informationssysteme: Dbis Repository