[optimize] PrecedingFilter: K-bounded sliding window for preceding::*[K] (#2129 follow-up to PR #6325)

exist-core-jmh/pom.xml

@@ -59,6 +59,10 @@
             <artifactId>lucene-core</artifactId>
             <version>${lucene.version}</version>
         </dependency>
+        <dependency>
+            <groupId>net.sf.xmldb-org</groupId>
+            <artifactId>xmldb-api</artifactId>
+        </dependency>
         <dependency>
             <groupId>org.openjdk.jmh</groupId>
             <artifactId>jmh-core</artifactId>

exist-core-jmh/src/main/java/org/exist/xquery/PrecedingAxisBenchmark.java

@@ -0,0 +1,156 @@
+/*
+ * eXist-db Open Source Native XML Database
+ * Copyright (C) 2001 The eXist-db Authors
+ *
+ * info@exist-db.org
+ * http://www.exist-db.org
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+package org.exist.xquery;
+
+import org.exist.test.ExistEmbeddedServer;
+import org.exist.xmldb.XmldbURI;
+import org.openjdk.jmh.annotations.*;
+import org.xmldb.api.DatabaseManager;
+import org.xmldb.api.base.Collection;
+import org.xmldb.api.base.Database;
+import org.xmldb.api.modules.XQueryService;
+
+import java.util.concurrent.TimeUnit;
+
+/**
+ * JMH benchmark for the {@code preceding::*} half of issue #2129.
+ *
+ * <p>craigberry's #2129 follow-up reproduced position-dependence for the
+ * wildcard preceding axis on a 50,000-element flat document: a query at
+ * {@code @xml:id='45000'} took roughly twice as long as the same query at
+ * {@code @xml:id='25000'}. The K-bounded sliding window in
+ * {@code LocationStep.PrecedingFilter} caps the retained match set at K,
+ * eliminating the unbounded accumulation that produced the late-position
+ * tax.</p>
+ *
+ * <p>This benchmark exercises the wildcard-vs-sibling and early-vs-late
+ * comparisons that the original mixed-purpose JUnit class measured with
+ * {@code System.nanoTime} and median-of-N. JMH handles statistical
+ * aggregation natively; the correctness assertions live in the
+ * companion XQSuite test {@code exist-core/src/test/xquery/preceding-axis.xql}.</p>
+ */
+@State(Scope.Benchmark)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@Warmup(iterations = 3, time = 1)
+@Measurement(iterations = 5, time = 2)
+@Fork(1)
+public class PrecedingAxisBenchmark {
+
+    private static final String LARGE_DOC = "/db/words-large.xml";
+
+    @Param({"5000", "25000", "45000"})
+    private int refPosition;
+
+    private ExistEmbeddedServer existServer;
+    private Database database;
+    private Collection root;
+    private XQueryService xqs;
+
+    /** Default constructor for JMH harness. */
+    public PrecedingAxisBenchmark() {
+    }
+
+    /**
+     * Boots an embedded eXist server, registers the XML:DB driver, and stores
+     * a 50,000-element flat words document used by the benchmark queries.
+     *
+     * @throws Exception if the embedded server fails to start, the database
+     *     driver cannot be registered, or the corpus document cannot be stored
+     */
+    @Setup(Level.Trial)
+    public void setUp() throws Exception {
+        existServer = new ExistEmbeddedServer(true, true);
+        existServer.startDb();
+
+        final Class<?> cl = Class.forName("org.exist.xmldb.DatabaseImpl");
+        database = (Database) cl.getDeclaredConstructor().newInstance();
+        database.setProperty("create-database", "true");
+        DatabaseManager.registerDatabase(database);
+        root = DatabaseManager.getCollection(XmldbURI.LOCAL_DB, "admin", "");
+        xqs = root.getService(XQueryService.class);
+
+        xqs.query(
+                """
+                let $words := for $i in (1 to 50000) return <w xml:id="{$i}">{$i}</w>
+                return xmldb:store('/db', 'words-large.xml', document {<words>{$words}</words>})
+                """);
+    }
+
+    /**
+     * Removes the corpus document, closes the test collection, and shuts down
+     * the embedded server.
+     *
+     * @throws Exception if removing the corpus document, closing the
+     *     collection, or stopping the embedded server fails
+     */
+    @TearDown(Level.Trial)
+    public void tearDown() throws Exception {
+        try {
+            xqs.query("xmldb:remove('/db', 'words-large.xml')");
+        } finally {
+            root.close();
+            DatabaseManager.deregisterDatabase(database);
+            existServer.stopDb(true);
+        }
+    }
+
+    /**
+     * Wildcard preceding axis with a positional predicate gated by a self::w
+     * filter. Pre-fix this accumulated every preceding match from doc start;
+     * post-fix the sliding window caps retention at K=5.
+     *
+     * @return the result-set size, returned so JMH's blackhole prevents the
+     *     call being optimized away
+     * @throws Exception if the embedded query fails
+     */
+    @Benchmark
+    public long wildcardPrecedingWithPositionalPredicate() throws Exception {
+        return xqs.query(
+                ("""
+                xquery version "3.1";
+                let $w := doc('%s')//w[@xml:id='%d']
+                return for $i in (1 to 5) return $w/preceding::*[5 + 1 - $i][self::w]/text()
+                """).formatted(LARGE_DOC, refPosition)
+        ).getSize();
+    }
+
+    /**
+     * preceding-sibling::w[K] baseline: walks the persistent sibling chain
+     * directly rather than the full preceding axis. Used as a relative
+     * lower-bound to interpret the wildcard preceding number.
+     *
+     * @return the result-set size, returned so JMH's blackhole prevents the
+     *     call being optimized away
+     * @throws Exception if the embedded query fails
+     */
+    @Benchmark
+    public long precedingSiblingBaseline() throws Exception {
+        return xqs.query(
+                ("""
+                xquery version "3.1";
+                let $w := doc('%s')//w[@xml:id='%d']
+                return for $i in (1 to 5) return $w/preceding-sibling::w[$i]/text()
+                """).formatted(LARGE_DOC, refPosition)
+        ).getSize();
+    }
+}

exist-core/src/main/java/org/exist/xquery/LocationStep.java

@@ -38,6 +38,8 @@
 import javax.xml.stream.XMLStreamException;
 import javax.xml.stream.XMLStreamReader;
 import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.Deque;
 
 /**
  * Processes all location path steps (like descendant::*, ancestor::XXX).
@@ -920,7 +922,7 @@ private Sequence getPrecedingOrFollowing(final XQueryContext context, final Sequ
                         final NodeProxy root = new NodeProxy(this, node);
                         final StreamFilter filter;
                         if (axis == Constants.PRECEDING_AXIS) {
-                            filter = new PrecedingFilter(test, root, next, result, contextId);
+                            filter = new PrecedingFilter(test, root, next, result, contextId, position);
                         } else {
                             filter = new FollowingFilter(test, root, next, result, contextId, position);
                         }
@@ -1417,12 +1419,19 @@ public boolean accept(final XMLStreamReader reader) {
     private class PrecedingFilter extends AbstractFilterBase {
         final NodeProxy root;
         final NodeProxy referenceNode;
+        // Sliding window of the most recent {@code limit} matches. Non-null only
+        // when limit > 0 (positional predicate {@code [K]} present). The K-th
+        // preceding element in axis order is the (K-th-from-end) match in doc
+        // order, so any match earlier than the K most recent cannot be selected
+        // and may be discarded as new ones are found.
+        final Deque<NodeProxy> window;
 
         PrecedingFilter(final NodeTest test, final NodeProxy root, final NodeProxy referenceNode, final NodeSet result,
-                final int contextId) {
-            super(test, result, contextId, -1);
+                final int contextId, final int limit) {
+            super(test, result, contextId, limit);
             this.root = root;
             this.referenceNode = referenceNode;
+            this.window = limit > 0 ? new ArrayDeque<>(limit) : null;
         }
 
         @Override
@@ -1431,11 +1440,16 @@ public boolean accept(final XMLStreamReader reader) {
 
             if (reader.getEventType() == XMLStreamReader.END_ELEMENT) {
                 // exited the root element, so  stop filtering
-                return currentId.getTreeLevel() != root.getNodeId().getTreeLevel();
+                if (currentId.getTreeLevel() == root.getNodeId().getTreeLevel()) {
+                    flushWindow();
+                    return false;
+                }
+                return true;
             }
 
             final NodeId refId = referenceNode.getNodeId();
             if (currentId.compareTo(refId) >= 0) {
+                flushWindow();
                 return false;
             }
 
@@ -1449,10 +1463,26 @@ public boolean accept(final XMLStreamReader reader) {
                         proxy.addContextNode(contextId, referenceNode);
                     }
                 }
-                result.add(proxy);
+                if (window != null) {
+                    if (window.size() == limit) {
+                        window.pollFirst();
+                    }
+                    window.addLast(proxy);
+                } else {
+                    result.add(proxy);
+                }
             }
             return true;
         }
+
+        private void flushWindow() {
+            if (window != null) {
+                for (final NodeProxy proxy : window) {
+                    result.add(proxy);
+                }
+                window.clear();
+            }
+        }
     }
 
 }

exist-core/src/test/xquery/optimizer/positional.xqm

@@ -176,7 +176,7 @@ function ot:optimize-simple-following-nested() {
 
 declare
     %test:stats
-    %test:assertXPath("not($result//stats:optimization[@type eq 'POSITIONAL_PREDICATE'])")
+    %test:assertXPath("$result//stats:optimization[@type eq 'POSITIONAL_PREDICATE']")
 function ot:optimize-simple-preceding() {
     let $w := doc($ot:DOC)//w[@xml:id='25000']
     return

exist-core/src/test/xquery/preceding-axis.xql

@@ -0,0 +1,132 @@
+(:
+ : eXist-db Open Source Native XML Database
+ : Copyright (C) 2001 The eXist-db Authors
+ :
+ : info@exist-db.org
+ : http://www.exist-db.org
+ :
+ : This library is free software; you can redistribute it and/or
+ : modify it under the terms of the GNU Lesser General Public
+ : License as published by the Free Software Foundation; either
+ : version 2.1 of the License, or (at your option) any later version.
+ :
+ : This library is distributed in the hope that it will be useful,
+ : but WITHOUT ANY WARRANTY; without even the implied warranty of
+ : MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ : Lesser General Public License for more details.
+ :
+ : You should have received a copy of the GNU Lesser General Public
+ : License along with this library; if not, write to the Free Software
+ : Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ :)
+xquery version "3.1";
+
+(:~
+ : Correctness regression tests for the {@code preceding::*} half of issue #2129.
+ : The K-bounded sliding window in {@code LocationStep.PrecedingFilter} must
+ : preserve the original axis semantics: ancestor exclusion, axis-order
+ : positional predicates, and identical result counts/values regardless of
+ : where the reference node sits in the document.
+ :
+ : The companion performance benchmark lives in
+ : {@code exist-core-jmh/.../PrecedingAxisBenchmark.java}.
+ :)
+module namespace pa = "http://exist-db.org/xquery/test/preceding-axis";
+
+declare namespace test = "http://exist-db.org/xquery/xqsuite";
+
+declare variable $pa:collection := "/db/preceding-axis-test";
+declare variable $pa:small-doc := $pa:collection || "/words-small.xml";
+declare variable $pa:tiny-doc := $pa:collection || "/tiny.xml";
+declare variable $pa:ancestors-doc := $pa:collection || "/ancestors.xml";
+
+declare
+    %test:setUp
+function pa:setup() {
+    let $col := xmldb:create-collection("/db", "preceding-axis-test")
+    let $small :=
+        document {
+            <words>{
+                for $i in (1 to 50) return <w xml:id="{$i}">{$i}</w>
+            }</words>
+        }
+    let $tiny :=
+        document {
+            <words>{
+                for $i in (1 to 10) return <w xml:id="{$i}">{$i}</w>
+            }</words>
+        }
+    let $ancestors :=
+        document {
+            <root><a/><b><c><d><target/></d></c></b></root>
+        }
+    return (
+        xmldb:store($col, "words-small.xml", $small),
+        xmldb:store($col, "tiny.xml", $tiny),
+        xmldb:store($col, "ancestors.xml", $ancestors)
+    )
+};
+
+declare
+    %test:tearDown
+function pa:cleanup() {
+    xmldb:remove($pa:collection)
+};
+
+declare
+    %test:assertEquals("5 6 7 8 9")
+function pa:reproducer-output-at-early-position() {
+    let $w := doc($pa:small-doc)//w[@xml:id = "10"]
+    return
+        string-join(
+            for $i in (1 to 5) return $w/preceding::*[5 + 1 - $i][self::w]/text(),
+            " "
+        )
+};
+
+declare
+    %test:assertEquals("20 21 22 23 24")
+function pa:reproducer-output-at-midpoint() {
+    let $w := doc($pa:small-doc)//w[@xml:id = "25"]
+    return
+        string-join(
+            for $i in (1 to 5) return $w/preceding::*[5 + 1 - $i][self::w]/text(),
+            " "
+        )
+};
+
+declare
+    %test:assertEquals("40 41 42 43 44")
+function pa:reproducer-output-at-late-position() {
+    let $w := doc($pa:small-doc)//w[@xml:id = "45"]
+    return
+        string-join(
+            for $i in (1 to 5) return $w/preceding::*[5 + 1 - $i][self::w]/text(),
+            " "
+        )
+};
+
+(:~
+ : {@code preceding::} excludes ancestors of the reference node even though
+ : they precede the reference in document order. Only the sibling {@code a}
+ : precedes {@code target} on the preceding axis.
+ :)
+declare
+    %test:assertEquals("a")
+function pa:preceding-excludes-ancestors() {
+    for $n in doc($pa:ancestors-doc)//target/preceding::*
+    return name($n)
+};
+
+(:~
+ : From {@code w[5]} on a flat doc of 10 elements, the k-th preceding element
+ : in axis order (reverse document order) is {@code w[5-k]}.
+ :)
+declare
+    %test:assertEquals("4", "3", "2", "1")
+function pa:preceding-positional-predicate-returns-kth-closest() {
+    let $w := doc($pa:tiny-doc)//w[@xml:id = "5"]
+    return
+        for $k in 1 to 4
+        return $w/preceding::*[$k]/text()
+};