EMF Model Query Developer Guide
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EMF Model Query Developer Guide > Programmer's Guide

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.

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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();