find siblings is known to be sorted

IS IT ? Need more confirmation from @adamretter, but it appears to work for this workload. Take forward and we can discuss later..

Author: alanpaxton

exist-core/src/main/java/org/exist/dom/persistent/NewArrayNodeSet.java

@@ -498,6 +498,13 @@ private boolean checkSorted() {
         return true;
     }
 
+    @Override
+    public void setKnownSorted(final boolean mergeContexts) {
+        isSorted = true;
+        removeDuplicates(mergeContexts);
+        updateDocs();
+    }
+
     @Override
     public void sort(final boolean mergeContexts) {
         if(isSorted) {

exist-core/src/main/java/org/exist/dom/persistent/NodeSet.java

@@ -386,4 +386,6 @@ boolean matchAncestorDescendant(NodeSet al, int mode, boolean includeSelf,
 
     void setTrackMatches(boolean track);
 
+    default void setKnownSorted(final boolean mergeContexts) {
+    }
 }

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

@@ -883,6 +883,7 @@ protected Sequence getSiblings(final XQueryContext context, final Sequence conte
                                         + "'");
                     }
                     currentSet = index.findElementsByTagName(ElementValue.ELEMENT, docs, test.getName(), null, this);
+                    currentSet.setKnownSorted(true);
                     currentDocs = docs;
                     registerUpdateListener();
                 }