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EMF Model Query Developer Guide
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Pruning the Search Tree

An EMF dataset is naturally hierarchically structured. Queries frequently traverse this tree content in search of objects. One technique for omptimizing queries is to detect when a branch of the tree cannot contain positive results, so that it may be skipped entirely. This technique is called pruning.

Prune Handlers

Any EObjectCondition can be configured with a PruneHandler that tells the condition when to prune the search tree. The default prune handler for a condition is NEVER which, as the name suggests, never prunes.

Prune Handler API
[ as SVG]

A query will check after each object that is encountered, and either accepted or rejected, whether to prune that object's subtree.

// search for books by Dickens, only in branches in the west end of the city
// because those are geographically closest

// case-insensitive substring match (hopefully we don't have a Weston Branch East)
final EObjectCondition isWest = new EObjectAttributeValueCondition(
        EXTLibraryPackage.Literals.LIBRARY__NAME,
        new SubStringValue("west", false);

PruneHandler pruner = new PruneHandler() {
        public boolean shouldPrune(EObject object) {
            // any branch not in the west cannot contain branches that are.
            // Also, anything that isn't a library can't contain branches
            return !isWest.isSatisfied(object);
        }};

// look for libraries in the west.  Wrap the condition that we already have
// so that we can apply the prune-handler.  Note that this wrapping is only
// necessary because our prune-handler uses the same condition
EObjectCondition libraryCond = new EObjectConditionAdapter(isWest, pruner);

// find books by Dickens
Writer dickens = getWriter("Charles Dickens");
EObjectCondition bookCond = new EObjectReferenceValueCondition(
        EXTLibraryPackage.Literals.BOOK__AUTHOR, isDickens);

// search from the root of the library hierarchy
Library mainBranch = getMainBranch();

// the nested query with the prune handler gets the western libraries to search
IQueryResult result = new SELECT(
    new FROM(new SELECT(
        new FROM(mainBranch), new WHERE(libraryCond))),
    new WHERE(bookCond)).execute();

Copyright (c) 2000, 2007 IBM Corporation and others. All Rights Reserved.


 
 
  Published under the terms of the Eclipse Public License Version 1.0 ("EPL") Design by Interspire